JP2898066B2 - Optical device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. 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 capable of switching without any change and transmitting and receiving. This optical device is optical CATV, optical
It can be applied to LAN and centralized optical measurement system.

[Summary of the Invention] The present invention relates to a device capable of switching light transmitted by a single mode optical fiber as it is as an optical signal, and includes at least one branch interference type modulator on a substrate such as LiNbO ₃ or LiTaO _3. And two asymmetrical forks,
A transmission refractive index control unit independent of an applied electric field is provided on one or both of the two branch paths constituting the light modulation unit, and further, a reflection unit and a light reception unit are provided on any of the input / output terminals. With the optical waveguide device configuration to modulate light having an arbitrary polarization plane, it is possible to easily manufacture an element having a light receiving and transmitting function,
It enables low-voltage driving, improvement of the reception SN ratio, variable reflection voltage, complementary output, wideband reception, and the like.

[Prior art] There are few examples of a reflection type optical switch having a reflection unit, and there is only an element having a wavelength filter in the reflection unit (Reference: EJMURPHY, J. OCENASEK, CRSANDAHL, RJL)
ISCO, AND YCCHEN; “Simultaneous Singl-Fiber Tran
s-mission of Video Bidirectinal Voice / Data Using
LiNbO3 Guided-Wave Devices, “Journal of lightwave
technology, vol. 6, No. 6 June 1988). FIG. 11 shows a conventional optical switch, Zcut-LiNbO ₃ optical waveguide 2, 2 'is provided on the substrate 1, electrode 3 and the reflecting portion 4 for applying an electric field in the Z-axis direction of the substrate crystal Is provided. The optical waveguides 2 and 2 'approach each other to form an optical directional coupler.
Turn on / off the optical input to That is, the optical device operates as a polarization independent optical switch.

[Problems to be Solved by the Invention] When a polarization-independent optical switching operation is realized in the directional coupling type, in order to realize a complete cross state, the shape (phase constant) of the optical waveguide and the coupling strength are required. , It is necessary to precisely match the three parameters of the length of the connection, which requires strict production conditions and high production accuracy. Further, the characteristics change depending on the difference in light wavelength and temperature. Each of the above-described directional coupling type polarization independent optical switches has drawbacks such as strict production conditions and a high operating voltage of 30 to 70V.

The present invention provides an optical device that solves the conventional drawbacks described above, has a simple configuration, can switch and modulate light at low voltage without depending on polarization, and further has a transmitting and receiving function. The purpose is to do.

Means for Solving the Problems In order to achieve the above object, the present invention comprises an optical waveguide formed on a substrate and having an asymmetric branch path and a branch interference type optical modulator, and the optical modulator. An electrode for applying an electric field to the two branches to perform a switching operation; an equivalent refractive index control unit provided on at least one of the two branches and independent of the applied voltage; and an input to the optical waveguide. And a light receiving section and a reflection section provided at one of the output end and the output end.

[Operation] The optical device according to the present invention is provided with an asymmetrical branch (an asymmetrical branch having different equivalent refractive indices of two branched optical waveguides and a branch interference type optical modulator) as optical waveguides. An equivalent refractive index controller which does not depend on an applied electric field is provided on one or both of the two branch paths constituting the unit, and a reflection unit and a light receiving unit are provided on any of the input / output terminals.

FIG. 1 is a diagram showing an example of such a configuration of the present invention. The optical waveguide 2 branches at a point P. Electrodes 3 are provided on both sides and in the middle of the parallel portions of the branch optical waveguides 2A and 2B. The optical waveguides 2A and 2B intersect at point O,
Optical waveguides 2C and 2D are formed on the extension. The distance between the optical waveguides 2A and 2B is wide, and a directional coupler is not formed. A branch interference type optical modulator 6 is formed at a portion from the incident side to the point O. The optical waveguides 2C and 2D have different equivalent refractive indices,
An asymmetric X branch 7 is formed on the left and right of the point O, and an asymmetric branch 8 is formed on the emission side from the point O. In this example, an example is shown in which the equivalent refractive index is changed by changing the width of 2C and 2D.
The method of forming the asymmetric branch path is not limited to this. Optical waveguide 2C
And the 2D output end are respectively provided with a reflecting section 4 and a light receiving section 5
Is provided, and an equivalent refractive index controller 9 is provided in the optical waveguide 2B. The equivalent refractive index controller 9 will be described later in detail. The reflecting section 4 and the light receiving section 5 may be connected to the output ends of the optical waveguides 2D and 2C, respectively, contrary to the illustration, and the equivalent refractive index control section 9 may be connected to the optical waveguide 2A or
It may be provided in both 2A and 2B.

FIG. 2 shows another configuration example of the present invention, in which the input side is an asymmetric branch path 8, the output side is a reflection-type branch interference type optical modulator 6 ', and the output ends of the optical waveguides 2A and 2B. Provided with a reflection section 4A. The reflecting portion 4A transmits a part of the incident light and guides it to the light receiving portion 5, and reflects a part of the incident light and returns it to the waveguides 2C and 2D.

The conventional configuration does not have this equivalent over-refractive-index control unit and controls the phase only by the electro-optic effect, so a polarization-independent optical switch could not be achieved without using a directional optical coupler structure. . In the present invention, switching can be performed without depending on polarization by changing the equivalent refractive index of one or both of the two optical waveguides of the modulation unit by the equivalent refractive index control unit. In addition, in the present invention, by further providing the reflection unit and the light receiving unit, transmission and reception can be performed without requiring a light source as a transmission source.

In the structure shown in FIG. 1, the light is switched to two optical paths of the asymmetric branch path by the switching. At this time, when the light is guided to the optical path including the reflection part, the light is reflected by the reflection part. Then, it follows the original waveguide and returns to the incident end. On the other hand, when the light is guided to an optical path having no reflecting portion, the light is received by the light receiving portion without being reflected. Therefore, on the one hand, a signal according to the switching operation enters the light receiving unit 5, and on the other hand, the return light is intensity-modulated by the switching operation and used as a transmission signal. In this case, when no modulation is performed, 100% of the light enters the light receiving section, so that the SN ratio of the received signal is improved. In addition, since the light receiving section can be directly coupled to one single mode waveguide to reduce the light receiving area, it is possible to receive a broadband signal (2 GHz or more). Although the amount of reflection can be changed by the reflectance of the material forming the reflecting portion, the feature of the present element is that the amount of reflection can be arbitrarily changed by the drive voltage.

In the case of the configuration shown in FIG. 2, the reflected light reflected by the reflector 4A is switched to two optical paths of an asymmetric branch path by a switching operation, and the reflected light can be used as a transmission signal. In this case, the outputs to the waveguides 2C and 2D are on,
Complementary outputs that turn off each other are obtained, so this element can be connected in cascade to exchange signals,
Applications such as optical CATV are possible. On the other hand, the outgoing light output to the light receiving unit 5 receives the signal light as it is regardless of the switching operation.

Next, the principle of switching operation of the present device that does not depend on polarization will be described in detail.

1. Principle of Operation The asymmetric branch is for separating the fundamental and first-order mode light generated at the X intersection O into a thick optical waveguide and a thin optical waveguide, respectively, and the branch interference type optical modulator is divided into two optical waveguides. This is because the light waves interfere with each other at the intersection O to generate the fundamental and first-order mode light. Further, the equivalent refractive index controller gives a certain 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 switching operation can be realized. The phase changes θ _E and θ _M of the TE / TM mode light when the equivalent refractive index controller and the voltage are applied are generally expressed as θ _E = a + αV θ _M = b + βV. a and b are phase changes of the TE mode light and the TM mode light by the equivalent refractive index control unit, and 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. The optical output P _o of one of the optical waveguides constituting the asymmetric branch of the optical device of the present invention is given by P _o = P _i sin ² (θ / 2) where P _i is the input light intensity. And the other optical waveguide optical output P _o 'is P _o' is given by ^{_{= P i cos 2 (θ /}} 2).

Here, θ = θ _E or θ _M. Therefore, the light output versus applied voltage characteristic curve of this device for TE / TM mode light depends on the phase changes a and b of the equivalent refractive index controller.

The operation of the device according to the present invention will be described by taking as an example a 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. In this case ^{_{α: β = n e 3 r}} 33: n o 3 r 13 ≒ 2.9: 1 ( although, n _o, n _e is ordinary index of refraction and the extraordinary refractive index of LiNbO ₃ crystal, r _33, r ₁₃ is the electro-optical (A Pockels constant), the optical output versus applied voltage characteristics of the TE mode light and the TM mode light are generally as shown in FIG. 3 (a). Since α and β have a relationship of almost an integer ratio, the equivalent refractive index control unit is appropriately configured so that the minimum points of the intensity of the TE and TM mode light substantially match as shown in FIG. 3 (b). By controlling a and b above, an optical switching operation independent of polarization can be achieved. In the device configuration shown in FIG. 1, in a device in which light propagates in the Z-axis direction of the LiNbO ₃ crystal and an electric field is applied in the Y-axis direction of the crystal, the proportional constants α and β have the same size and only the sign. Include different electro-optic coefficients, β = −α, and the light output versus applied voltage characteristic is
The periods match as shown in FIG. Now, the equivalent refractive index control unit is appropriately constructed, and as shown in FIG.
If a phase difference of = ± 2Nπ (N is an integer) is given, a polarization-independent optical switching operation is performed. The feature of this element configuration is that each output of the asymmetric branch path can be continuously changed by the applied electric field.

A specific configuration of the equivalent refractive index control unit for providing these phase differences is, for example, as follows.

1) The lengths of the two optical waveguides constituting the branch interference type optical modulator are made different from each other.

2) The refractive indices of the two optical waveguides constituting the branching interference type optical modulator are different from each other over the entire length or a part thereof.

3) The widths of the two optical waveguides forming the branch interference type optical modulator are different from each other over the entire length or a part thereof.

4) Equivalent to the two optical waveguides having a refractive index smaller than the refractive index of the optical waveguide in one or both of the two optical waveguides constituting the branching interference type optical modulator by loading a transparent insulating material. Different refractive indices.

And the like, and the above-described methods can be combined. When LiNbO ₃ or LiTaO ₃ is used as a substrate, the refractive index of the waveguide is generally controlled by the diffusion amount of Ti into the waveguide. By changing the thickness and / or width of the Ti film for forming the waveguide, one or both of the refractive index and the width of the waveguide can be controlled.

The methods 1) to 3) can generally give an appropriate phase change amount to TE / TM mode light. Method 4) is
In particular, a large phase change can be given to TM mode light. By combining the methods 1) to 4),
It is possible to realize an optical switching operation that does not depend on the polarization of the zero bias operation. For example, in the configuration shown in FIG. 1 described above, in an element that propagates light in the Z-axis direction of a LiNbO ₃ crystal and applies an electric field in the Y-axis direction of the crystal, a
It can be obtained by appropriately configuring the equivalent refractive index controller so as to satisfy both conditions of + b = ± 2Nπ and a = π / 2 ± 2Mπ (M is an integer).

It should be noted that the asymmetric Y-branch can be configured in exactly the same manner as the configuration of the equivalent refractive index control unit described above.

2. Embodiment Here, a LiNbO ₃ crystal is used as a substrate material, light is propagated in the Z-axis direction of the configuration shown in FIG. 1, more specifically, the LiNbO ₃ crystal, and an electric field is applied in the Y-axis direction. So-called Z-axis propagation Li
An optical switch according to the present invention configured on an NbO ₃ element will be described.

As shown in FIG. 5, the optical waveguide 2 formed on the LiNbO ₃ substrate 11 branches at the point P into the waveguides 2A and 2B to form a branch interference type optical modulator. The waveguides 2A and 2B intersect at a point O to form an asymmetrical branch. A reflection section 4 is provided at one end of the waveguide 2C, and a light receiving section 5 is connected to the other waveguide 2D. ing. Electrodes 3 are provided on both sides of the waveguides 2A and 2B of the light modulation section and in the middle thereof, and a cladding layer 10 is provided as an equivalent refractive index control section. This cladding layer
Reference numeral 10 corresponds to a fourth method for forming the above-described equivalent refractive index control unit, and includes a photoresist (refractive index n = 1.
61) is applied and solidified by heating to form the transparent cladding layer 10.

FIG. 6 shows the relationship between the register length l and the static phase difference (Δφ = a + b) between the TE mode and the TM mode received by the light receiving section 5, and a straight line A constitutes the branch interference type modulation section shown in FIG. The straight line B shows the characteristics when the resist is loaded on the lower optical waveguide 2B, and the straight line B shows the characteristics when the resist is loaded on the upper optical waveguide 2A. Each of them changes almost linearly. Even when the resist is not loaded, there is a static phase difference (about 0.7π), which is caused by the two optical waveguides 2A and 2 constituting the branch interference type optical modulator.
This is due to the difference in the branch length of B, and is equivalent to applying the above-described configuration 1) in advance. In any case, a polarization-independent optical switch can be constructed by appropriately selecting the resist length. When the phase difference Δφ = 0, that is, l = 6 mm (in FIG. 5, the upper optical waveguide 2A of the two branch paths is loaded with a resist having a length of 6 mm), an optical switch independent of polarization can be formed. FIG. 7 shows TE mode light and TM when l = 6 mm.
9 is a graph showing light output versus applied voltage characteristics of each waveguide (2C, 2D) of the asymmetric branch for mode light. It shows almost the same characteristics for both TE / TM mode light. That is, it is understood that the optical switch operates as an optical switch independent of polarization. It can be seen that the half-wave voltage is as low as 16V. As described above, in the Z-axis propagation element configuration, an optical switching operation that can be driven at a low voltage and does not depend on polarization can be easily obtained.

In Figure 5, as the substrate, LiNbO ₃ Nikae, LiTaO ₃
The effect is the same even if is used.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

Embodiment 1 FIG. 8 shows an embodiment of the optical device according to the present invention.
This embodiment is a device corresponding to the configuration shown in FIGS. 1 and 5. Although the electrodes for performing the switching operation and the equivalent refractive index controller are not shown, they are as described above. In the following embodiments, the electrodes and the equivalent refractive index controller are not shown.

In the present embodiment, a single mode optical fiber 12 is
A photodiode 13 such as a pin-PD (photodiode) or an APD (avalanche photodiode) is attached to one of the emission ends of the asymmetric branch path, and a complete reflection film 14 such as a thick Al vapor-deposited film is attached to the other. Therefore, as described above, a signal corresponding to the switching operation by applying a voltage to an electrode (not shown) is output to the photodiode 13, while the intensity-modulated light can be extracted from the incident side.

Embodiment 2 FIG. 9 shows another embodiment of the optical device according to the present invention. This embodiment has a structure in which an asymmetric branch path is provided on the input side, and a reflection-type branch interference type light modulator is formed between the perfect reflection film 14 and the asymmetric branch path. As described with reference to FIG. 2, complementary outputs whose ON and OFF are opposite to each other are emitted from the single-mode optical fiber 12 for incidence and the single-mode or multi-mode optical fiber 15 for emission. The modulation section is a reflection-type branch interference type light modulation section, and the driving voltage can be greatly reduced.

In this embodiment, since the interval between the asymmetric branch paths is only 100 μm or less, the output light is guided to the photodiode 13 by the single mode or multimode optical fiber 15.

Embodiment 3 FIG. 10 shows another embodiment of the present invention. This embodiment is the second
This is an example corresponding to the configuration shown in the figure. That is, as the reflecting portion, a thin Al vapor-deposited film 14A is used, or the crystal end face itself is used as a reflecting surface, and a part of incident light is reflected, a part is transmitted, and a photodiode 13 provided immediately after the reflecting film 14A is provided. Lead to. Single-mode optical fibers 12, 12A are attached to the asymmetric branch end on the incident side. The operation of this embodiment is as described with reference to FIG.

[Effects of the Invention] As described above, the present invention has the following effects.

1) As a method for obtaining the switching function, an asymmetric branch path is used instead of an optical directional coupler, so that manufacturing conditions and manufacturing accuracy are not strict. Therefore, the design is easy,
Easy to fabricate. Further lowering of the voltage is possible.

2) The optical switching operation independent of the polarization can be formed only by providing the equivalent refractive index control unit, and the configuration is simple.

3) In particular, the device of the present invention, which is configured on a so-called Z-axis propagating LiNbO ₃ crystal that propagates light in the Z-axis direction of a LiNbO ₃ crystal and applies an electric field in the Y-axis direction, has less damage to light and has a Ti Sometimes, there is no external diffusion of Li ₂ O, and the fundamental mode optical waveguide can be easily formed. Furthermore, since each output of the asymmetric branch path, that is, the reflected light signal can be continuously changed by the applied electric field, analog light modulation is possible.

4) Since it has a reflecting portion, a light source for transmission is unnecessary.
In addition, this device is equipped with a light receiving section, so this element
It can be regarded as an element in which multi-functions of light reception, transmission and modulation are integrated into one. Therefore, reliability and economy can be improved.

5) It is possible to arbitrarily change the amount of reflection of the reflection unit by the modulation voltage, which facilitates system design and the like.

6) Because of the switching configuration, two complementary outputs can be obtained at the same time, making the system application extremely flexible.

7) When no modulation is performed, 100% of the light is incident on the light receiving unit, so that the SN ratio of the received signal is improved.

8) Since the light receiving section receives light emitted from one single mode waveguide, the light receiving area can be reduced, and an optical band signal can be received.

[Brief description of the drawings]

1 and 2 are diagrams showing the basic configuration of the device of the present invention, FIG. 3 is a diagram showing an example of the light output versus applied voltage characteristic of the device of the present invention, and FIG. FIG. 5 is a diagram showing an example of an applied voltage characteristic, FIG. 5 is a diagram showing an embodiment of the present invention, FIG. 6 is a diagram showing a photoresist length vs. static phase difference characteristic in the embodiment of FIG. 5, and FIG. FIG. 8 is a diagram showing an optical output versus applied voltage characteristic of a polarization independent optical switch formed by the embodiment of FIG. 5, FIG. 8 to FIG. 10 are diagrams each showing an embodiment of the device of the present invention, FIG. FIG. 9 is a diagram illustrating a configuration of a conventional optical switch. _{1 ... Zcut-LiNbO 3, 2,2} '... optical waveguide, 3 ... electrode 4 ...
Reflector, 5 light receiver, 6 branch interference light modulator, 7 asymmetric X branch, 8 asymmetric branch, 9 equivalent refractive index controller, 10 cladding layer, 11 LiNbO ₃ substrate, 12 ... Single mode optical fiber, 13 ... Photodiode, 14 ... Complete reflection film, 14
A: Incomplete reflection film, 15: Optical fiber.

Claims (6)

(57) [Claims] An optical waveguide formed on a substrate and having an asymmetric branch and a branch interference type optical modulator, and a switching operation is performed by applying an electric field to two branches constituting the optical modulator. An equivalent refractive index control unit provided on at least one of the two branch paths and independent of the applied voltage; a light receiving unit and a reflection unit provided on one of the input and output terminals of the optical waveguide. An optical device comprising a part. 2. The optical device according to claim 1, wherein the input side of the optical waveguide is an asymmetric branch path, and the output side is a reflection-type branch interference light modulator. 3. The apparatus according to claim 2, wherein the reflecting section is a perfect reflector, two optical fibers are attached to the asymmetrical branch end on the input side, and a light receiving section is attached to one of the optical fibers. The optical device according to claim 2. 4. The reflecting section transmits a part of incident light, a light receiving section is attached immediately after the reflecting section, and two single mode optical fibers are attached at an asymmetric branch end on the incident side. The optical device according to claim 2, wherein: 5. The substrate according to claim 1, wherein the substrate is a LiNbO ₃ or LiTaO ₃ crystal, light propagates in the Z-axis direction of the crystal, and an electric field is applied in the Y-axis direction. An optical device according to any one of the above. 6. The substrate according to claim 1, wherein the substrate is a LiNbO ₃ or LiTaO ₃ crystal, light propagates in the X-axis direction of the crystal, and an electric field is applied in the Z-axis direction. An optical device according to any one of the above.