JP2812974B2 - Polarization independent optical switch - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an optical signal processing device for optical communication and optical measurement, and more particularly to an optical signal transmitted by a single mode optical fiber depending on its polarization. The present invention relates to a waveguide-type optical device that switches without power.

(Summary of the Invention) The present invention relates to a device for switching light transmitted through a single-mode optical fiber in the form of an optical signal, comprising: a branch interference type optical modulator on a substrate such as lithium niobate or lithium tantalate. At least one asymmetric Y-branch unit is provided at the subsequent stage of the branch interference type optical modulator, and a static phase independent of the applied electric field is provided to the branch interference type optical modulator in addition to the dynamic phase control unit which depends on the applied electric field. A control unit is provided.

Thus, a polarization-independent optical switch that switches the optical path of light having an arbitrary polarization mode is realized and can be operated at a low voltage.

(Prior art) Conventionally, this type of optical switch uses a LiNbO ₃ crystal as a substrate,
An electric field is applied in the Z-axis direction of the crystal. In this case LiNbO ₃
Since the refractive index differs depending on the TE mode light and the TM mode light propagating through the waveguide on the substrate, the conventional optical switch and the standard single mode optical fiber cannot be directly coupled. For this reason, research on a polarization-independent optical switch that does not depend on polarization has recently been active. Hereinafter, the related art will be described.

As a conventional optical switch, as shown in FIG. 2, an optical directional coupling type optical switch (a) in which two optical waveguides are close to each other,
There are an asymmetric Y-branch optical switch (b) and a total reflection optical switch (c) having at least one asymmetric Y-branch whose optical waveguides have different equivalent refractive indices. Above all, optical directional coupling type optical switches are the mainstream.

The optical directional coupling type optical switch has one optical waveguide 1a or
Light energy is transferred to the other 1b or 1a by coupling of the leakage electric field of the guided light propagating through 1b. However, in order to realize a perfect cross state, advanced manufacturing techniques are required. The asymmetric Y-branch type optical switch utilizes the principle that the fundamental mode light and the primary mode light are separated by changing the equivalent refractive index of the two optical waveguides on the output side, and is very simple to design and manufacture. It has the feature of. Although the total reflection type optical switch has a very simple structure, it is difficult to induce a high refractive portion at the intersection of the optical waveguide by an applied electric field.

Many of these optical switches are so-called polarization-dependent optical switches that can use only specific linearly polarized light. On the other hand, the conventional examples of the polarization-independent optical switch are limited to very few examples, and most of them are optical directional coupling type optical switches, and examples thereof are the following literatures (i) and (ii).
There is. As an asymmetric Y-branch optical switch, reference (ii)
i), and these polarization independent optical switches are all L
It is made of an iNbO ₃ crystal, and applies an electric field in the Z-axis direction of the crystal.

References (i) K. Kondo, Y. Ohta, Y. Tanisawa, T. Aoyama, R. Ishikaw
a: Low-Drive-Voltage and Low-Loss Polarization-
Independent LiNbO ₃ Optical Waveguide Switches, Electron Lett.,
Vol.23, No.21, P.1167,1987 (ii) JEWatson, MAMilbrodt and TCRice: A Polar
ization-Independent1 × 16Guided-Wave Optical Swit
ch Integrated on LiNbO ₃ (Polarization independent 1 × 16 optical switch), Journal of Ligh
twave Technology, Vol. LT-4, No. 11, P. 1717, Nov. 1986 (iii) Y. Silberberg, P. Perlmutter and JEBaran: Dig
ital optical switch, Appl.Phy
s. Lett., 51 (16), 19, P. 1230, Oct. 1987 (Problems to be Solved by the Invention) It is difficult for a directional coupling type polarization independent optical switch to realize a perfect cross state. In addition, it is necessary to precisely match the three parameters of the shape (phase constant) of the optical waveguide, the strength of the coupling, and the length of the coupling, so that strict production conditions and high production precision are required. Further, the characteristics change depending on the difference in light wavelength and temperature. The directional-coupling type polarization-independent optical switches described in Documents (i) and (ii) have drawbacks such as strict production conditions and a high operating voltage of 30 to 70 V. Further, although an asymmetric Y-branch type polarization independent optical switch described in the literature (iii) has been studied, a large drawback is that the operating voltage is high.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an asymmetric Y-branch type polarization-independent optical switch which solves the above-mentioned problems, has low manufacturing conditions and precision, and is easy to design.

(Means for Solving the Problems) In order to achieve this object, a polarization-independent optical switch according to the present invention is an optical switch for switching the optical path of light having an arbitrary polarization mode. And a single asymmetric Y-branch unit at least at the subsequent stage of the branch interference-type light modulation unit, wherein the two asymmetrical Y-branch optical waveguides have different equivalent refractive indices. In addition to the dynamic phase control unit having an electro-optic effect depending on an applied electric field, one or both of the optical waveguides of the branch interference type optical modulator A static phase control unit for controlling an optical modulation operation point independent of an applied electric field so as to simultaneously minimize the optical output of the optical switch of light having different polarization modes at a predetermined applied electric field. The light modulation operating point is set.

(Operation) That is, in the configuration of the conventional optical switch, the phase control is performed only by the electro-optic effect without the static phase control unit, and it is difficult to realize the polarization-independent optical switch. In the configuration of the present invention, a static phase control unit is provided to give a static phase change to the guided light without depending on the electro-optic effect. Thus, it is easy to perform optical switching without depending on polarization. Furthermore, according to the optical switch of the present invention, the polarization-independent optical switch and other devices can be easily configured on the same substrate, and the voltage can be reduced, so that key devices for future optical fiber communication and optical information processing will be used. Can be

(Examples) Hereinafter, the present invention will be described in detail by way of examples with reference to the accompanying drawings.

FIG. 1 shows the basic configuration of the optical switch of the present invention. The element shown in FIG. 3A has a fundamental mode optical waveguide 6 on the input side, a branch interference type optical modulator 3 and an asymmetric Y branch 4 on the output side, and the modulator has a static phase change in the guided light. It is provided with a static phase control unit 5 for giving. The element shown in FIG. 1B has a configuration in which one optical waveguide of the input shown in FIG. 1A is replaced with an asymmetric Y branch.

The asymmetric Y branch path 4 separates the fundamental and first-order mode light generated at the X intersection O into a thick optical waveguide and a thin optical waveguide, respectively. The branch interference type optical modulator 3 includes a light wave split into two optical waveguides. At the intersection O to generate the fundamental and first-order mode light.
Further, the static phase controller 5 gives a constant phase difference between the TE mode light and the TM mode light. By controlling this phase difference to an appropriate value, a polarization independent optical switch can be realized.

The phase changes θ _E and θ _M of the TE / TM mode light when the static phase controller and the voltage are applied are generally expressed as θ _E = a + αV (1) θ _M = b + βV (2). a and b are phase changes of the TE mode light and the TM mode light by the static phase control unit. The second term on the right side of each equation is converted into the TE mode light and the TM mode light by the electro-optic effect based on the application of the voltage (V). The induced phase change. α and β are proportional constants depending on the electro-optic coefficient. Optical output P ₀₁ of one of the optical waveguides constituting the asymmetric Y branch of the optical device of the present invention is given by P ₀₁ = P _i sin ² When the input light intensity _{P i (θ / 2) (} 3). The optical output P ₀₂ of the other optical waveguide is given by ^{_{P 02 = P i cos 2 (}} θ / 2) (4). However, it is θ = θ _E or θ _M.
Therefore, the modulation operating point of the optical output vs. applied voltage characteristic of this element with respect to TE / TM mode light is determined by the phase change a,
Depends only on b.

As an example, the operation of the device according to the present invention will be described with reference to the case where light is propagated along the X axis of the LiNbO ₃ crystal and an electric field is applied in the Z axis direction of the crystal in the configuration of FIG. ^{_{α: β = n e 3 γ}} 33: n o 3 γ 13 ≒ 2.9: 1 (5) provided that n _o, since n _e is given by the ordinary index of refraction and the extraordinary refractive index of LiNbO ₃ crystal, TE mode light and TM The light output versus applied voltage characteristic of mode light is generally given as shown in FIG. Since α and β have a relationship of almost an integer ratio, the static phase control unit is appropriately controlled, and as shown in FIG.
If the a and b are controlled so that the minimum points of the intensity of the M-mode light substantially coincide, a polarization-independent optical switch is obtained.

In the element configuration shown in FIG. 1A, light propagates in the Z-axis direction of the LiNbO ₃ crystal and applies an electric field in the Y-axis direction of the crystal. Are equal and only the sign is different, β
= −α, and the light output versus applied voltage characteristics have the same period as shown in FIG. 4 (a). Now, by appropriately controlling the static phase control unit, as shown in FIG. 4 (b), a + b = ±
When a phase difference of 2Nπ (N is an integer) is given, a polarization independent optical switch is obtained. The feature of this element configuration is that each output of the symmetric Y branch path can be continuously changed by an applied electric field.

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

The length of one or both of the two optical waveguides constituting the branching interference type optical modulator is changed.

The width of one or both of the two optical waveguides constituting the branching interference type optical modulator is changed.

The thickness of the Ti film forming one or both of the two optical waveguides constituting the branching interference type optical modulator is changed.

A transparent insulating material having a refractive index smaller than the refractive index of the optical waveguide is loaded on one or both of the two optical waveguides constituting the branching interference type optical modulator.

 There are such means.

The means from (1) can generally give an appropriate amount of phase change to TE / TM mode light. In particular,
A large phase change can be given to TM mode light. By combining the following means, it is possible to realize a polarization-independent optical switch with zero bias operation. For example, in the element shown in FIG. 1A which transmits light in the Z-axis direction of the LiNbO ₃ crystal and applies an electric field in the Y-axis direction of the crystal, a + b = ± 2Nπ and a = π / It can be obtained by appropriately controlling the static phase control unit so as to satisfy both conditions of 2 ± 2Mπ (M is an integer).

In the device shown in FIG. 1 (a), when light is input from an asymmetric branch and light is output from one fundamental mode optical waveguide, it can be used as a two-input one-output optical switch. That is, in the element having the characteristics shown in FIG. 3B, only light propagating through one optical waveguide of the asymmetrical branch can be extracted by applying an appropriate voltage.

Further, also in the device having the configuration shown in FIG. 1B, when the TE / TM mode light is input to only one optical waveguide of the asymmetric branch on the input side, the same function as the device shown in FIG. can do. Also, when TE / TM mode light is input to both of the asymmetric branch paths on the input side, it is possible to configure a matrix optical switch having two inputs and two outputs.

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

Example 1 using a LiNbO ₃ crystal substrate material in this example, to propagate light in the Z axis direction of the crystal, an electric field is applied to the Y-axis direction, the present invention constructed in a so-called Z-propagation LiNbO ₃ on the element Will be described.

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

FIG. 6 shows the static phase difference (Δ
φ = a + b), wherein a straight line 9 corresponds to the lower optical waveguide 1a and two straight lines 10 correspond to the upper optical waveguide 1b constituting the branch interference type optical modulator shown in FIG. This is a characteristic when a photoresist is loaded. Each of them changes almost linearly. Even when the photoresist is not loaded, there is a static phase difference (approximately 0.7π). This is due to the difference in the branch length between the two optical waveguides 1a and 1b constituting the branch interference type optical modulator. This corresponds to performing the operation in advance. In any case, a polarization-independent optical switch can be formed by appropriately selecting the photoresist length. When the phase difference Δφ = 0, that is, l = 6 mm (in FIG. 5, the upper optical waveguide 1b of the two branches is loaded with a photoresist having a length of 6 mm), a polarization-independent optical switch can be formed.

FIG. 7 shows the optical output versus applied voltage characteristics of the respective outputs (a) and (b) of the asymmetric Y branch for the TE mode light and the TM mode light when l = 6 mm. It shows almost the same characteristics for both TE / 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 V.

As described above, according to the present invention, the optical switch and the matrix optical switch which can be directly coupled with the single mode optical fiber very indispensably for future optical fiber communication can be constituted.

Embodiment 2 FIG. 8 (a) shows that, in the configuration of the present invention, the lengths (l ₁ and l ₂ ) of _two optical waveguides constituting a branch interference type optical modulator as a static phase controller are changed. 1 is an embodiment of a polarization-independent optical switch realized by the present invention.

Embodiment 3 FIG. 8 (b) shows the configuration of the present invention by changing the Ti film thickness (t ₁ , t ₂ ) forming the optical waveguide of the branching interference type optical modulator particularly as a static phase controller. 9 is an example of a polarization independent optical switch realized.

Fourth Embodiment FIG. 8 (c) shows the configuration of the present invention by changing the widths (w ₁ , w ₂ ) of two optical waveguides constituting a branch interference type optical modulator as a static phase controller. 9 is an example of a polarization independent optical switch realized.

Fifth Embodiment FIG. 8 (d) shows the length of two optical waveguides constituting a branch interference type optical modulator when a plurality of operations are performed in the configuration of the present invention, particularly as a static phase controller. l ₁ and l ₂ )
And an embodiment of a polarization-independent optical switch realized by loading an insulating material on a part of the optical waveguide.

(Effects of the Invention) As described above in detail, according to the present invention, there are the following various advantages.

First, since the optical switch according to the present invention is not an optical directional coupler but an asymmetric Y-branch optical switch, the manufacturing conditions and precision are not strict and the design is easy, so that the manufacturing is easy. Since the polarization-independent optical switch can be formed only by providing the static phase control unit, the configuration is simple. If the manufacturing conditions are appropriately selected, a polarization-independent optical switch with zero bias operation can be constructed, so that no bias voltage is required.

In particular, a polarization-independent optical switch formed on a so-called Z-axis propagating LiNbO ₃ crystal that propagates light in the Z-axis direction of the LiNbO ₃ crystal and applies an electric field in the Y-axis direction has less damage to light.
There is no external diffusion of Ti ₂ O at the time of Ti diffusion, and a fundamental mode optical waveguide can be easily formed. Further, since each output of the asymmetric branch path can be continuously changed by the applied electric field, analog light modulation becomes possible.

In particular, the Z-axis propagating LiNbO ₃ element avoids the influence of the natural birefringence of the LiNbO ₃ crystal, so phase matching is easy, and the TE / TM mode optical separator can be used simply by providing a static phase controller. Can be formed on the same substrate together with the optical waveguide type optical device of (1), and a high-performance device in the future can be realized (elements having different functions can be integrated).

[Brief description of the drawings]

FIG. 1 shows a basic configuration diagram of an optical switch of the present invention, FIG. 2 shows various configuration diagrams of a conventional optical switch, and FIGS. 3 and 4 show optical output versus application of an optical switch element of the present invention. FIG. 5 shows an embodiment of the optical switch of the present invention. FIG. 6 and FIG. 7 show photoresist length versus static position in the embodiment shown in FIG. FIG. 8 shows phase difference characteristics and optical output versus applied voltage characteristics, respectively. FIG. 8 shows some embodiments of the optical switch of the present invention. 1, 1a, 1b ... optical waveguide, 2 ... electrode 3 ... branch interference type light modulator 4 ... asymmetric Y branch, 5 ... static phase controller 6 ... input light, 7, 7a, 7b …… Output light 8 …… Clad layer, 11… Loaded insulating material

Continuation of the front page (56) References JP-A-62-160405 (JP, A) JP-A-63-147145 (JP, A) JP-A-63-147146 (JP, A) JP-A-63-116118 (JP, A) JP-A-62-39826 (JP, A) JP-A-58-202406 (JP, A) JP-A-63-33710 (JP, A) U.S. Pat. No. 4,291,939 (US, A) European publication 267708 (EP, A1) Hiroshi Nishihara, Masamitsu Haruna, Toshiaki Suhara, “Optical Integrated Circuit” (Ohm), Optics Letters, Vol. 7 No. 11 pp. 549-551 (1982) (58) Field surveyed (Int. Cl. ⁶ , DB name) G02F 1/00-1/313 G02B ^6/ 12-6/14

Claims (8)

(57) [Claims] 1. An optical switch for switching an optical path of light having an arbitrary polarization mode, wherein the optical switch has an optical waveguide configuration including at least a branch interference type optical modulator and at least a subsequent stage of the branch interference type optical modulator.
And two asymmetric Y branches, wherein the asymmetric Y branch is Y
An asymmetric Y branch in which the equivalent refractive indices of the two branched optical waveguides are different, wherein the branch interference type optical modulator further includes a dynamic phase controller having an electro-optic effect dependent on an applied electric field, One or both of the optical waveguides of the branching interference type optical modulator has a static phase control unit that controls an optical modulation operation point that does not depend on an applied electric field, and the light of polarization modes different from each other at a predetermined applied electric field. The polarization-independent optical switch, wherein the optical modulation operating point is set so as to simultaneously minimize the optical output of the optical switch. 2. An optical switch according to claim 1, wherein said static phase control section comprises said branching interference type optical modulation section.
A polarization-independent optical switch, characterized in that it is a control unit for changing the length of two optical waveguides. 3. The optical switch according to claim 1, wherein said static phase controller is a controller which changes an equivalent refractive index of one or both of the two optical waveguides of said branching interference type optical modulator. A polarization independent optical switch. 4. The optical switch according to claim 3, wherein said static phase control section has a refractive index smaller than a refractive index of an optical waveguide of said branching interference type optical modulation section, and comprises a transparent insulating material. A polarization-independent optical switch, which is a control unit loaded on a road. 5. The polarization-independent optical switch according to claim 3, wherein said static phase control section is a control section for changing an optical waveguide width of said branching interference type optical modulation section. 6. An optical switch according to claim 3, wherein said static phase control section is a control section for changing a titanium film thickness forming an optical waveguide of said branching interference type optical modulation section. Dependent light switch. 7. The substrate of the optical switch according to claim 1, wherein the substrate material is lithium niobate or lithium tantalate which transmits light in the Z-axis direction of the crystal and applies an electric field in the Y-axis direction. A polarization independent optical switch. 8. The substrate of the optical switch according to claim 1, wherein the substrate material is lithium niobate or lithium tantalate which transmits light in the X-axis direction of the crystal and applies an electric field in the Z-axis direction. A polarization independent optical switch.