JP4987335B2 - Optical device - Google Patents

Description

  The present invention relates to an optical device, and more particularly to an optical device having a function of monitoring the intensity of light propagating through an optical waveguide.

  In an optical communication system in which a signal is placed on light and transmitted through an optical fiber, various functional elements that perform various processes necessary for light transmission are used. These functional elements can obtain desired characteristics with high accuracy by monitoring the output light and performing feedback control of the operation of the elements based on the monitoring results.

For example, an optical intensity modulator in which a Mach-Zehnder type optical waveguide, a modulation electrode, and a bias electrode are formed on a substrate such as lithium niobate (LiNbO ₃ ; hereinafter referred to as LN) as a functional element that applies intensity modulation to an optical signal Is being used. In this optical intensity modulator, it is necessary to perform modulation with reference to a state in which the phase difference between two optical waves to be combined is 0 and π at the multiplexing point of the Mach-Zehnder optical waveguide (the bias at the modulation operating point). In order to obtain this reference phase state, the modulated signal light is monitored to adjust the voltage applied from the bias electrode.

  Conventionally, an optical waveguide device with a monitor described in Patent Document 1 is known as a device for realizing such an optical monitor. FIG. 4 shows a schematic configuration of a cross section of the optical waveguide device with a monitor. In the figure, an optical waveguide 102 is formed on a substrate 101, and an evanescent component introduction layer 200 having a higher refractive index than that of the optical waveguide 102 is provided on the upper portion (substrate surface). The photodetector 204 is disposed on the optical waveguide 102 via the evanescent component introduction layer 200.

In the optical waveguide device with a monitor shown in FIG. 4, the light propagating in the P direction in the drawing through the optical waveguide 102 propagates in a state where a part of the light oozes out of the optical waveguide 102 as an evanescent component. The evanescent component penetrates into the photodetector 204 through the evanescent component introduction layer 200, and only a part of the evanescent component reaches the light receiving surface 205 of the photodetector 204. Thus, the propagation light is monitored by detecting a part of the evanescent component.
JP 2001-215371 A

  However, in the above-described optical waveguide device with a monitor, the entry angle of the evanescent component into the photodetector 204 is shallow at about 5 degrees with respect to the propagation direction P, so that most of the infiltrated light is within the photodetector 204. It is dissipated and only a small part can reach the light receiving surface 205. As a result, there is a problem that the monitor efficiency (light receiving sensitivity) in the photodetector 204 is extremely low, and practical performance cannot be obtained.

  The present invention has been made in view of the above points, and an object of the present invention is to make it possible to efficiently monitor the intensity of light propagating through an optical waveguide. The structure of the monitor portion is simple and easy to mount. To provide a device.

  The present invention has been made to solve the above-mentioned problems, and the invention according to claim 1 includes an optical waveguide element in which an optical waveguide is formed on a substrate, and an evanescent component of light propagating through the optical waveguide. And an optical medium arranged on the surface of the substrate so that optical coupling occurs, a light introducing means for taking the combined light and guiding it to a light detection means, and reflecting the taken light to An optical device comprising: an optical path conversion unit that converts an optical path into a direction of a light receiving surface of the light detection unit; and the light detection unit that receives light after the optical path conversion.

  According to the present invention, the evanescent component of the light propagating in the optical waveguide is coupled to the optical medium constituting the light introducing means, and the coupled light travels through the medium at an angle close to parallel to the substrate, and is reflected by the optical path changing means. The optical path is bent and further proceeds in the direction of the light receiving surface of the light detection means, and is received by the light detection means. Therefore, most of the light taken into the light introducing means is received by the light detecting means via the optical path changing means, so that the light propagating through the optical waveguide can be monitored efficiently. Further, since the light detection means can be installed face down with the light receiving surface facing the substrate, mounting is easy.

  Further, the invention according to claim 2 is the optical device according to claim 1, wherein the optical path changing means is a boundary surface between the light introducing means having a predetermined inclination angle with respect to the substrate and another medium, The total reflection condition for the light taken into the light introducing means is satisfied at the boundary surface.

According to the present invention, the light combined with the light introducing means is totally reflected by the boundary surface (optical path changing means) and proceeds to the light detecting means, so that no light loss occurs in the optical path changing means. Therefore, the monitoring efficiency is good. In addition, since this boundary surface can be formed by polishing the end face of a light introducing means (predetermined uniform optical medium), the structure is simple, and this part (light introducing means and optical path changing means) is manufactured and mounted. Is easy.
Note that the total reflection condition is satisfied when the refractive index of the light introducing means and the other medium is given and the incident angle of the light beam on the boundary surface is equal to or greater than a predetermined angle (critical angle). That means.

  According to a third aspect of the present invention, in the optical device according to the first or second aspect, a buffer layer having a refractive index lower than that of the optical waveguide is provided between the optical waveguide and the light introducing means. The light introducing means has a higher refractive index than the buffer layer.

  According to the present invention, since the evanescent component is coupled to the light introducing means via the buffer layer having a low refractive index, only an appropriate amount of light (signal light) propagating through the optical waveguide is extracted as monitor light. Thus, it is possible to suppress an increase in loss with respect to signal light. That is, by appropriately designing the refractive index, film thickness, and coupling length (length of the portion in contact with the optical waveguide) of the buffer layer, the amount of light taken into the light introducing means can be adjusted.

  According to the present invention, it is possible to efficiently monitor the intensity of light propagating through an optical waveguide. As a result, for example, the bias adjustment of the modulation operation point in the light intensity modulator can be realized with high accuracy.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
1 and 2 show a configuration of an optical device according to an embodiment of the present invention. FIG. 1 is a schematic external view of the optical device, and FIG. 2 is a propagation direction (P direction) of the optical waveguide. FIG. In FIG. 1, the buffer layers (the buffer layer 103 and the monitor buffer layer 203) are omitted for simplification. In the present embodiment, the optical device is configured by mounting a monitor unit 20 on an optical waveguide element 10 using an LN substrate.

The optical waveguide element 10 includes an LN substrate 101, an optical waveguide 102 formed on the LN substrate 101 by thermal diffusion of titanium (Ti), an electrode 104 for applying a modulation electric field to the optical waveguide 102, and an optical waveguide 102. And a buffer layer 103 of silicon oxide (SiO ₂ ) for preventing the light wave propagating through the electrode 104 from receiving absorption loss by the electrode 104.

  Here, the waveguide pattern of the optical waveguide 102 is described as being a straight waveguide as shown in FIG. 1, but may be a waveguide having another configuration. For example, a Mach-Zehnder type optical waveguide may be used. The optical waveguide 102 below the electrode 104 receives a modulated electric field, induces a refractive index change due to the electro-optic effect, and gives a phase change to the light wave passing through that portion. The configuration of the electrode 104, such as the material, film thickness, and electrode shape, is a matter designed using a well-known technique in accordance with the function and characteristics of the optical waveguide element 10, and therefore will not be described in detail here.

  The buffer layer 103 is formed with a predetermined film thickness over the entire surface of the LN substrate 101. The film thickness is selected to be sufficiently thick (up to several μm) so that light wave absorption loss does not occur in the electrode 104 as described above. However, the portion where the monitor unit 20 is mounted is formed with a film thickness that optimizes the amount of light taken into the monitor unit 20 as will be described later (the buffer layer in this portion is referred to as the monitor unit buffer layer 203). I will call it).

  The monitor unit 20 includes a photodetector 204 that detects the monitored light and a light introducing unit 201 that takes in an evanescent component of the light propagating through the optical waveguide 102 and guides it to the photodetector 204.

The light introducing unit 201 is a block-shaped optical medium, and a material having a refractive index higher than that of the monitor unit buffer layer 203 is selected. For example, in the present optical device, SiO ₂ is used as the monitor unit buffer layer 203 as described above, but the refractive index n _{SiO 2} is about 1.5, and the refractive index of the light introducing unit 201 is about 3.5. Some silicon (Si) is used. It is assumed that the refractive index distribution in the light introducing portion 201 is uniform (that is, no optical waveguide is formed inside and light propagates freely).

  The bottom surface and top surface of the light introducing portion 201 are optically polished with a surface roughness of about the wavelength of signal light (for example, λ = 1.55 μm) or less, and the light introducing portion 201 is in contact with the surface of the monitor portion buffer layer 203. Is installed. On the upper surface, the photodetector 204 is placed face down with the light receiving surface 205 facing the substrate. The height of the light introduction unit 201 needs to be higher than the height of the optical path conversion unit 202, but there is no particular upper limit value for the height, and it may be set arbitrarily.

  A V-groove is formed on the bottom surface of the light introducing portion 201 in a direction perpendicular to the waveguide propagation direction (P direction in FIG. 2), and the slope on the front side (left side in FIG. 2) is the optical path. It functions as the conversion unit 202. Specifically, the evanescent component that has entered the light introducing unit 201 from the optical waveguide 102 through the monitor unit buffer layer 203 is defined by the angle of entry (the angle formed with the waveguide propagation direction P) in the light introducing unit 201. ) Freely propagates at about 5 degrees and enters the V-groove slope of the optical path conversion unit 202. Then, by being reflected by the optical path conversion unit 202, the traveling direction is bent upward (perpendicular to the substrate) where the photodetector 204 is located.

  Accordingly, it is desirable that the inclination angle of the optical path conversion unit 202 (slope) is set to an angle of about 40 to 45 degrees with respect to the substrate surface so that the propagation direction of the light after the optical path conversion is upward, that is, toward the light receiving surface 205. However, since the light receiving surface 205 of the photodetector 204 has a certain spread, it is sufficient if the inclination angle is such that light after the optical path conversion can enter the light receiving surface 205. That is, the inclination angle of the optical path conversion unit 202 can be set in consideration of the opening of the light receiving surface 205 and the height of the light introducing unit 201.

  The formed V-groove portion is filled with air or a predetermined adhesive. A vacuum may be used without filling them. However, when an adhesive is used, the refractive index thereof is lower than that of the light introducing portion 201.

  Depending on the medium filled in the V-groove portion, the total reflection condition can be satisfied with respect to the incident evanescent component in the optical path conversion unit 202 which is a boundary surface between the medium and the light introducing unit 201. . When the total reflection condition is satisfied, the incident light is reflected 100% and input to the photodetector 204, so that the monitoring efficiency can be greatly improved.

For example, in the case where the light introducing unit 201 is configured by Si exemplified above (refractive index n _Si = 3.5) and the V groove portion is filled with air (refractive index n _air = 1), in the optical path converting unit 202 The critical angle θ _c, which is the minimum incident angle where total reflection occurs, is
θ _c = sin ⁻¹ (n _air / n _Si ) ≈17 (degrees)
It is. Therefore, when the inclination angle of the optical path conversion unit 202 with respect to the substrate surface is 40 to 45 degrees as described above, the incident angle of the evanescent component to the optical path conversion unit 202 is larger than the critical angle, so that total reflection occurs, and light detection The monitor 204 can capture the monitor light efficiently.

  In the optical device of the present embodiment, the monitor unit buffer layer 203 provided below the light introducing unit 201 has a role of adjusting the amount of the evanescent component that is combined with the light introducing unit 201 and taken in. That is, if the monitor buffer layer 203 does not exist and the optical waveguide 102 and the light introducing portion 201 are in direct contact, the intensity distribution of the propagation mode light is higher in the refractive index in the optical waveguide 102 below the light introducing portion 201. The amount of light combined with the light introducing portion 201 increases due to the deviation toward the light introducing portion 201 side. However, this means that the propagation loss increases when viewed from the signal light propagating through the optical waveguide 102, which is not preferable.

On the other hand, when the monitor part buffer layer 203 is provided (when the material is SiO ₂ , the refractive index is n _SiO2 = 1.5 <n _e ), the intensity distribution of the propagation mode light has a low refractive index. This suppresses the displacement toward the light introducing portion 201 and reduces the evanescent component that oozes out to the light introducing portion 201. This effect can be adjusted by controlling the refractive index, the film thickness, and the coupling length of the monitor unit buffer layer 203. Therefore, by designing these parameters according to the amount of light required for the optical monitor, the propagation loss of signal light can be minimized.

  As described above, according to the present embodiment, the evanescent component taken into the light introducing unit 201 is bent in the direction perpendicular to the substrate 101 by the total reflection on the slope of the V groove (optical path conversion unit 202), and the light The light is received by the detector 204. Thus, the evanescent component extracted from the signal light can be used as monitor light with high efficiency without being dissipated in the middle.

  Further, in this embodiment, the amount of light that oozes out from the optical waveguide 102 to the light introducing portion 201 as an evanescent component is adjusted by the monitor portion buffer layer 203. Thereby, it is possible to realize an excellent optical device that achieves both improvement in monitoring sensitivity and reduction in propagation loss of signal light.

As described above, the embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to the above, and various design changes and the like can be made without departing from the scope of the present invention. It is possible to
For example, when the total reflection condition is not satisfied by the optical path conversion unit 202, a reflective film of a metal film or a dielectric multilayer film can be formed on the slope of the V groove to increase the reflectance at the optical path conversion unit 202. It is.

Moreover, the monitor part buffer layer 203 can be substituted with an adhesive for adhering the light introducing part 201 to the surface of the substrate, or a composite structure of a buffer layer such as SiO ₂ and an adhesive. However, this adhesive must have the optical characteristics described above.
When an adhesive is used for the monitor part buffer layer 203 (or a part thereof), the light introduction part 201 and the light introduction part 201 are controlled in order to control the film thickness of the adhesive that adjusts the amount of the evanescent component that exudes to the light introduction part 201. It is desirable to provide a spacer layer between them. For example, a gold (Au) thin film is formed in a predetermined film thickness of 0.1 to 1 μm to form a spacer layer, and an adhesive is filled in a portion without the spacer layer. This Au thin film may be formed on the surface on the optical waveguide element 10 side, or may be formed on the bottom surface side of the light introducing portion 201. Further, the bond length between the evanescent component and the adhesive may be controlled by adjusting the area of the Au thin film.

  Further, the optical path changing unit 202 may be formed in a structure other than the V-groove as long as it has a predetermined inclination angle with respect to the substrate. For example, a pit formed in a pyramid shape (a quadrangular pyramid shape). It can also consist of one slope.

Next, an embodiment of the present invention will be described with reference to FIGS.
FIG. 3 shows a planar configuration of an optical device according to an embodiment of the present invention in which a plurality of monitor units are mounted on the optical SSB modulator.

  The optical SSB modulator 30 is provided with sub-Mach-Zehnder optical waveguides 302a and 302b on both arms of the main Mach-Zehnder optical waveguide 301, respectively, and the phase difference between the sub-Mach-Zehnder optical waveguides 302a and 302b is π / 2. In addition to performing intensity modulation by the modulation signal, a bias electric field is applied to each Mach-Zehnder optical waveguide, and each modulation operating point has a phase difference π in the sub-Mach-Zehnder optical waveguides 302a and 302b, and a phase difference π in the main Mach-Zehnder optical waveguide 301. It is an optical modulator that is driven so as to be / 2 or -π / 2. By such a driving method, SSB (Single Side Band) modulation for outputting only one of the sidebands generated by the modulation is performed.

  In order to perform the SSB modulation with high accuracy, it is necessary to accurately adjust the modulation operation point by bias control. Therefore, in the present embodiment, as shown in FIG. 3, the monitor unit 20 is provided immediately after the output unit of each Mach-Zehnder optical waveguide.

The manufacturing method of the optical device according to this example is as follows.
A resist is applied to the surface of the LN substrate 101, and the optical waveguide pattern shown in FIG. 3 is formed by exposure and development. Then, 800 nm of titanium (Ti) is deposited on the entire surface by vapor deposition, and the titanium is removed by lift-off leaving the portion of the optical waveguide pattern. Thereafter, the entire substrate is heated at 1000 ° C. for 10 hours to thermally diffuse the optical waveguide pattern titanium into the substrate. Thereby, the optical waveguide 102 of the optical SSB modulator is formed.

Further, a SiO ₂ film having a thickness of 1 μm is formed as the buffer layer 103 on the entire surface of the LN substrate 101. Then, the buffer layer 103 at the three monitor unit 20 installation portions shown in FIG. 3 is etched to a film thickness of 0.1 to 0.5 μm to form the monitor unit buffer layer 203.
Thereafter, gold (Au) is deposited to a thickness of 30 μm by electrolytic plating to form predetermined modulation electrodes and bias electrodes. For simplicity, these electrodes are omitted in FIG.

On the other hand, the light introducing part 201 is manufactured by forming a V-groove in a Si substrate having a substrate thickness of 0.25 mm and then cutting out into a predetermined dimension to form a chip. Here, a square shape with a side of 400 μm was used, and a PD (photodiode) chip (photodetector 204) with a side of 300 μm was placed.
Further, dicing was used to form the V groove, and the inclination angle of the V groove slope was 45 degrees. In addition to the dicing process, a known technique such as anisotropic chemical etching can also be applied.

  The chip of the light introducing unit 201 thus obtained is placed on the monitor unit buffer layer 203 of the optical SSB modulator 30 with the V-groove portion on the lower side. Further, the photodetector 204 is placed face down on the light introducing unit 201. For fixing these parts, an adhesive that is transparent to light having a wavelength of 1.5 μm is used.

  Thus, the optical SSB modulator 30 in which the monitor unit 20 is mounted on the output unit of each Mach-Zehnder optical waveguide is completed. The optical SSB modulator 30 performs bias control by feedback by separately monitoring the outputs of the main and sub Mach-Zehnder optical waveguides 301, 302a, and 302b, and can realize highly accurate SSB modulation.

1 is an external view of an optical device according to an embodiment of the present invention. It is a cross-sectional block diagram of the optical device of FIG. It is one Example of the optical device which mounted the monitor part in the optical SSB modulator. It is a cross-sectional block diagram of the conventional optical waveguide device with a monitor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Optical waveguide element 20 ... Monitor part 30 ... Optical SSB modulator 101 ... LN board | substrate 102 ... Optical waveguide 103 ... Buffer layer 104 ... Electrode 200 ... Evanescent component introduction | transduction layer 201 ... Light introduction part 202 ... Optical path conversion part 203 ... Monitor part Buffer layer 204... Photodetector 205.

Claims (2)

An optical waveguide element having an optical waveguide formed on a substrate;
A light introducing means configured of an optical medium disposed on the surface of the substrate so as to be optically coupled with an evanescent component of light propagating through the optical waveguide, and taking the coupled light and guiding it to a light detecting means; ,
A buffer layer in contact with both of the optical waveguide and the light introducing means, provided between the optical waveguide and the light introducing means, and having a lower refractive index than the optical waveguide and a lower refractive index than the light introducing means; ,
An optical path converting means for reflecting the light taken into the light introducing means and converting the optical path thereof toward the light receiving surface of the light detecting means;
The light detection means for receiving the light after the optical path conversion;
I have a,
The material of the optical medium constituting the light introducing means is silicon,
The optical device is characterized in that the optical path changing means is a slope of a V-groove formed on the surface of the light introducing means on the buffer layer side . The optical device according to claim 1, wherein the V groove is formed by anisotropic chemical etching .

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