JP2011075920A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator that can reliably perform separation into signal light and radiation light. <P>SOLUTION: The optical modulator includes a substrate 1 having an electro-optical effect; optical waveguides (2 to 4) formed on the substrate; and a modulating electrode for modulating a lightwave propagating the optical waveguides. The optical waveguide has at least a multiplexing part 3 for multiplexing the lightwave propagating the plurality of the optical waveguides 2; and an output waveguide 4 guided from the multiplexing part to an end portion of the substrate; two radiation light beams 5, emitted from the multiplexing part, are guided to the substrate end portion 10 by a slab waveguide or three-dimensional waveguide; in the output waveguide 4, the radiation light emitted from the multiplexing part crosses an optical path to the substrate end portion 10; an end portion of the output waveguide is arranged on the outside of the two radiation light beams in the substrate end portion; and the angle of a smaller side, among intersection angles formed by the output waveguide and radiation light, is ≥3°. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical modulator, and more particularly, to an optical modulator having a multiplexing unit that multiplexes light waves propagating through a plurality of optical waveguides, and having a configuration for deriving output light from the multiplexing unit to the outside.

  In the optical communication field and the optical measurement field, an optical modulator using a substrate having an electro-optic effect such as lithium niobate is frequently used. In these optical modulators, an optical waveguide and a modulation electrode for modulating an optical wave propagating through the optical waveguide are formed on a substrate having an electro-optic effect.

  The optical waveguide that constitutes the optical modulator uses an interference optical system called a Mach-Zehnder type waveguide, which combines a light wave that propagates through a plurality of optical waveguides (branch waveguides) Are provided with an output waveguide extending from the multiplexing section for extracting signal light (combined light).

  Further, since a plurality of light waves interfere with each other in the multiplexing section, higher-order mode light is generated, and two radiated lights are emitted into the substrate on both sides of the output waveguide. These emitted lights are also used as detection signals used for bias control in order to suppress the drift phenomenon of the optical modulator.

  Since the signal light emitted from the output waveguide and the two radiated lights exist close to each other at the end of the substrate constituting the optical modulator, it is essential to separate them so as not to mix them. In Patent Document 1, a proposal has been made to suppress interference with radiated light by bending an output waveguide.

  Moreover, in patent document 2, in order to suppress the reflection of the light wave in the edge part of a board | substrate, the refractive index difference of the optical waveguide in a board | substrate and an optical fiber is utilized, and Snell's law is satisfy | filled at a board | substrate edge part. It has also been proposed to bend the optical waveguide.

  On the other hand, an optical modulator having a substrate thickness of 30 μm or less has been proposed for the purpose of reducing the driving voltage of the optical modulator. In such an optical modulator, light confinement in the thickness direction of the substrate is strong, but the light confinement force is weak in the lateral direction of the substrate, and the emitted light propagates in a flat shape spreading in the lateral direction. .

  As a result, on the substrate end face, the signal light and the radiated light partially overlap each other, and problems such as deterioration of the extinction ratio and a phase difference between the signal light and the radiated light when the radiated light is monitored become apparent. Therefore, separation of signal light and radiated light is a more important problem.

International Publication (WO) 2005/124438 JP 2006-47956 A

  The problem to be solved by the present invention is to provide an optical modulator capable of solving the above-described problems and reliably separating signal light and radiated light.

  In order to solve the above problem, an invention according to claim 1 includes a substrate having an electro-optic effect, an optical waveguide formed on the substrate, and a modulation electrode for modulating a light wave propagating through the optical waveguide. In the optical modulator, the optical waveguide includes at least a multiplexing unit for multiplexing the light waves propagating through the plurality of optical waveguides, and an output waveguide led out from the multiplexing unit to the substrate end. Then, the two radiated lights emitted from the multiplexing part are led to the end of the substrate by a slab waveguide or a three-dimensional waveguide, and the output waveguide has a radiated light emitted from the multiplexing part. The end of the output waveguide is disposed outside the two radiated lights at the substrate end across the optical path to the substrate end, and the angle of intersection between the output waveguide and the radiated light The smaller angle is 3 degrees or more.

  The invention according to claim 2 is the optical modulator according to claim 1, wherein the thickness of the substrate in the portion where the output waveguide is formed is 4 to 30 μm.

  According to a third aspect of the present invention, in the optical modulator according to the third aspect, the light receiving means for detecting the radiated light detects the radiated light at a position away from the end of the output waveguide. It is arranged.

  According to the invention of claim 1, in an optical modulator comprising a substrate having an electro-optic effect, an optical waveguide formed on the substrate, and a modulation electrode for modulating a light wave propagating through the optical waveguide, The optical waveguide has at least a multiplexing part for multiplexing the light waves propagating through the plurality of optical waveguides, and an output waveguide led out from the multiplexing part to the substrate end, and from the multiplexing part The two emitted light beams are led out to the end of the substrate by a slab waveguide or a three-dimensional waveguide, and the output waveguide is used until the emitted light emitted from the combining unit reaches the end of the substrate. The end of the output waveguide is disposed outside the two radiated lights at the end of the substrate, and the angle of the smaller intersection angle between the output waveguide and the radiated light is 3 When the output waveguide crosses the propagation path of the emitted light, Shako is suppressed from again coupled to the output waveguide, it is possible to provide a signal light and radiation light and a can be reliably separated light modulator. In addition, the present invention only needs to adjust the shape of the output waveguide, and there is no fear of complicating the manufacturing process and increasing the manufacturing cost.

  According to the invention of claim 2, since the thickness of the substrate where the output waveguide is formed is 4 to 30 μm, the radiated light propagates flatly in the substrate, and the signal light and the radiated light are transmitted. Even in the case where it is easy to mix, signal light and radiated light can be reliably separated by applying the present invention. Further, the minimum thickness of the substrate was 4 μm from the viewpoint of the strength of the substrate.

  According to the invention of claim 3, since the light receiving means for detecting the emitted light is arranged to detect the emitted light at a position away from the end portion of the output waveguide, the light guiding means is connected to the light receiving means. It becomes possible to reliably prevent the signal light emitted from the waveguide from entering.

It is the schematic which shows the optical modulator of this invention.

Hereinafter, the optical modulator of the present invention will be described in detail below.
FIG. 1 is a schematic view showing an example of an optical modulator of the present invention, and is a plan view of the optical modulator as viewed from above. However, the modulation electrode is not shown in order to make the shape of the optical waveguide easy to see.

  The optical modulator of the present invention includes a substrate 1 having an electro-optic effect, an optical waveguide (2-4) formed on the substrate, and a modulation electrode for modulating an optical wave propagating through the optical waveguide. In the optical modulator, the optical waveguide includes a multiplexing unit 3 for multiplexing light waves propagating through the plurality of optical waveguides 2, and an output waveguide 4 led out from the multiplexing unit to the substrate end 10. The two radiated lights 5 emitted from the multiplexing section are led to the substrate end 10 by a slab waveguide or a three-dimensional waveguide, and the output waveguide 4 is supplied from the multiplexing section. The emitted radiated light 5 traverses the optical path leading to the substrate end 10, and the end of the output waveguide is disposed outside the two radiated lights 5 at the substrate end 10. The angle θ of the smaller intersection angle between the waveguide and the emitted light is 3 degrees or more, That.

  In FIG. 1, reference numeral 6 denotes an input optical fiber, reference numeral 7 denotes an output optical fiber, and reference numeral 8 denotes a light receiving means for detecting radiated light. The optical waveguide of FIG. 1 illustrates a Mach-Zehnder type waveguide, but the present invention is not limited to this, and there is a multiplexing unit that combines light waves propagating through a plurality of optical waveguides. The configuration of the present invention is applicable as long as it has an output waveguide for deriving signal light (combined light) from the wave section to the outside of the substrate.

  In FIG. 1, the radiated light propagates using the slab waveguide of the substrate, but may be configured to propagate using a three-dimensional waveguide. In the case of a slab waveguide, the beam diameter (spot diameter) gradually increases as the radiated light propagates. For this reason, the definition of the angle θ at which the output waveguide and the emitted light intersect in the present invention is ambiguous. In such a case, pay attention to a light wave having a light intensity that causes a defect such as a deterioration in the extinction ratio when recombining with the output waveguide in the spread radiation light. All the angles with the smaller crossing angle with the waveguide for use are set to be 3 degrees or more. As the direction of the output optical waveguide 4 when determining the angle θ, the tangential direction 9 of the waveguide at the intersection is used.

  The optical modulator of the present invention as shown in FIG. 1 suppresses recombination of the radiated light into the output waveguide when the output waveguide crosses the radiated light propagation optical path, so that the signal light and the radiation are emitted. It is possible to reliably separate light. In addition, the present invention only needs to adjust the shape of the output waveguide, and there is no fear of complicating the manufacturing process and increasing the manufacturing cost.

  As the substrate material having an electro-optic effect used in the optical modulator of the present invention, for example, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), quartz-based materials, and combinations thereof are used. Is possible. In particular, a lithium niobate (LN) crystal having a high electro-optic effect is preferably used.

  Since the optical modulator of the present invention can reliably separate the signal light and the radiated light, the thickness of the substrate in the portion where the output waveguide is formed is particularly 4 to 30 μm. It can use suitably for the optical modulator using the thin plate which has. In these thin plates, the radiated light propagates in a flat shape in the substrate, and the signal light and the radiated light are likely to be mixed, but the smaller angle of intersection between the output waveguide and the radiated light is 3 degrees or more. By maintaining, signal light and radiated light can be reliably separated.

  As a method for forming an optical waveguide on a substrate having an electro-optic effect, it can be formed by diffusing Ti or the like on the substrate surface by a thermal diffusion method or a proton exchange method. It is also possible to use a ridge-shaped waveguide having a convex portion corresponding to the optical waveguide, such as etching a substrate other than the optical waveguide or forming grooves on both sides of the optical waveguide.

  In order to verify the above condition of the crossing angle θ, an X-shaped cross waveguide is formed by Ti diffusion on an LN substrate having a substrate thickness of 8 μm, and a light wave is incident from one of the waveguides. The extinction ratio of the light wave traveling straight and exiting at the other end was measured. The mode diameter of light was set to 10 μm, and the crossing angle θ was set to 2.4, 2.8, 3, 4, and 5 degrees. The measurement results are shown in Table 1.

  As can be seen from Table 1, when the intersecting angle θ becomes shallow, a crosstalk (interference) phenomenon between the waveguides occurs, and the extinction ratio deteriorates. In a general optical modulator, the use limit of the extinction ratio is 10 dB. Therefore, when the angle θ is 3 degrees or more, the extinction ratio is 10 dB or more. In the “Evaluation” column of Table 1, those with an extinction ratio of 10 dB or more were indicated as appropriate (◯), and others were indicated as inappropriate (×).

The modulation electrode can be formed by forming a Ti / Au electrode pattern and forming an electrode thereon by a gold plating method or the like. Further, if necessary, a buffer layer such as a dielectric SiO ₂ may be provided on the substrate surface after the optical waveguide is formed, and a modulation electrode may be formed on the buffer layer.

  In the optical modulator of the present invention, as shown in FIG. 1, the light receiving means 8 for detecting the radiated light 5 is arranged to detect the radiated light 5 at a position away from the end of the output waveguide 4. Has been. Thereby, it is possible to reliably prevent the signal light emitted from the output waveguide 4 from entering the light receiving means 8, and it is possible to reliably detect only the radiated light.

  As described above, according to the present invention, it is possible to provide an optical modulator capable of reliably separating signal light and radiated light.

1 Substrate 2 Optical waveguide (branch waveguide)
3 Optical waveguide (multiplexing part)
4 Optical waveguide (output waveguide)
5 Synchrotron radiation 6 Optical fiber for incident light 7 Optical fiber for outgoing light 8 Light receiving means 9 Tangent direction of output waveguide 10 Edge of substrate

Claims (3)

In an optical modulator having a substrate having an electro-optic effect, an optical waveguide formed on the substrate, and a modulation electrode for modulating a light wave propagating through the optical waveguide,
The optical waveguide has at least a multiplexing unit for multiplexing light waves propagating through a plurality of optical waveguides, and an output waveguide led out from the multiplexing unit to the substrate end,
Two radiated lights emitted from the multiplexing part are led to the substrate end by a slab waveguide or a three-dimensional waveguide,
The output waveguide traverses the optical path from the radiated light emitted from the multiplexing unit to the substrate end, and the end of the output waveguide is outside the two radiated lights at the substrate end. Part is placed,
An optical modulator characterized in that the smaller angle of intersection between the output waveguide and the emitted light is 3 degrees or more.   2. The optical modulator according to claim 1, wherein a thickness of the substrate in a portion where the output waveguide is formed is 4 to 30 [mu] m.   4. The optical modulator according to claim 3, wherein the light receiving means for detecting the radiated light is arranged to detect the radiated light at a position away from the end of the output waveguide. Light modulator.

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