JP3681891B2 - Optical module - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical module used in the field of optical communication, for example, an optical module in which an optical waveguide device (optical waveguide body) such as an optical modulator and an optical switch is integrated with a light emitting element and a light receiving element. is there.
[0002]
[Prior art]
In recent years, in the field of wired communication, optical fiber communication systems have been put into practical use in transmission paths called relay systems that include an access system that connects between main transmission systems and communication stations, and communication capacity has been increased. Today, optical fiber communication is being introduced not only to relay systems but also to subscriber systems that connect offices and homes, and is a high-speed, broadband service that integrates telephone, facsimile, data communication, video transmission, etc. as an information highway. The use of advanced information communication by integrated digital network (ISDN) is developing.
[0003]
For this reason, it is predicted that a large amount of communications will be consumed due to the advanced use of ISDN, such as a large increase in information access by non-telephone systems other than telephones in the future. Will be.
[0004]
A waveguide type using the electro-optic effect of a ferroelectric substrate made of KDP (KH ₂ PO ₄ ), lithium niobate (LiNbO ₃ ), lithium tantalate (LiTaO ₃ ), etc. against the background of high-speed and broadband communication. Optical control devices (hereinafter referred to as optical waveguide devices) are promising. These optical waveguide devices are driven at a relatively low voltage, have a low loss, have a high response speed, and can be sufficiently modulated even in the millimeter wave band. In addition, since there is no chirping in principle, even an optical fiber having a large dispersion has a feature that high-capacity transmission can be performed with high quality.
[0005]
Conventionally, as shown in FIG. 5, the optical fiber is positioned and aligned with an optical fiber through a hermetically sealed optical semiconductor element and a lens system, and the same is hermetically sealed with a pigtail type optical semiconductor module 61 that is YAG welded. The optical fiber device chip 62 and the optical fiber module 62 are individually aligned with the input / output fiber via the functional optical device chip and the lens system, and YAG welded pigtail type functional optical device modules 62 are individually connected. What is connected by the member 63 is known.
[0006]
Here, as the package used, the optical semiconductor module 61 is generally a can package or a butterfly type package, and the functional optical device module 62 is a pigtail type using optical fibers 64 and 65 for input and output. Is generally used.
[0007]
As the optical fiber 64, functional optical devices are generally devices having polarization direction dependency. Therefore, the polarization plane is not provided on the emission side of the optical semiconductor module 61 or on the incident side of the LN optical modulator or optical switch. A storage fiber is used.
[0008]
Further, since the extinction ratio of the linearly polarized light after propagating through the polarization plane preserving fiber is deteriorated, the angle of the polarizer is generally adjusted on the incident side end face of the functional optical device module 62 and pasted with a UV adhesive or the like. Methods and others may improve the extinction ratio.
[0009]
[Problems to be solved by the invention]
However, in the above-described conventional configuration, each of the optical semiconductor module and the optical fiber, the optical fiber and the functional optical device module has a mode field mismatch, and the problem that the coupling efficiency deteriorates due to the design, Since there were two assembly adjustment points, which caused an increase in excess loss in the process, the efficiency of the entire module was bad, and the light intensity of the light source could not be extracted sufficiently from the functional optical device module. It was.
[0010]
Also, when the optical semiconductor module and the functional optical device module are connected to the polarization-maintaining fiber, if the angle adjustment of the polarization plane is complicated and the adjustment is wrong, the performance such as the extinction ratio of the functional optical device is adversely affected. And there was a problem that the assembly cost was high.
[0011]
In addition, the structure in which the optical semiconductor module and the functional optical device module are connected with the polarization-preserving fiber is not suitable for installation in the system because the space efficiency is poor and the mounting density is lowered. However, it took time to install and remove the module, and there was a problem that maintenance efficiency was poor.
[0012]
An object of the present invention is to overcome the above-mentioned problems, and to provide a small and highly reliable optical module capable of simple and highly efficient optical connection.
[0013]
[Means for Solving the Problems]
In order to solve the above-described problems, an optical module of the present invention includes an optical semiconductor element on one of the high position surfaces of a base on which a step having high position surfaces is formed on both sides of the low position surface, and the other of the high position surfaces. In addition, optical modulators having optical waveguides optically connected to the optical semiconductor element formed on the lower surface are respectively arranged face-down, and light is applied to the optical semiconductor element and the optical modulator on the lower surface. A condensing lens to be connected and an optical isolator are arranged, and the optical modulator is formed by forming the optical waveguide for guiding the TM mode on a substrate made of Z-cut lithium niobate, and the optical isolator Includes a first polarizer, a Faraday rotator, a second polarizer, and a half-wave plate whose optical axis is inclined by 22.5 ° from the polarization direction of the second polarizer from the optical semiconductor element side. do it And wherein the Rukoto.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments according to the present invention will be described in detail with reference to the drawings. 1A and 1B are a top view and a side view of the optical module M1, respectively. FIG. 2 shows an enlarged perspective view thereof. The optical module M1 includes bases 2 and 12, an optical modulator 1 that is an optical waveguide mounted thereon, an LD element 3 that is an optical semiconductor element, a ball lens 4 that is a condensing lens or a collimating lens, and an optical isolator. 5 and an optical fiber 6 or the like.
[0015]
Here, the optical modulator 1 includes a pyroelectric substrate made of, for example, lithium niobate (hereinafter referred to as LN), an optical waveguide formed on one main surface (lower surface), a control electrode, and the like. Is made of, for example, a non-doped silicon substrate, and its main surface is a (100) plane or a (110) plane so that anisotropic etching can be easily performed. For simplicity, the optical waveguide and the control electrode in the optical modulator 1 are not shown.
[0016]
The base 2 is composed of a V-groove 21, LD elements 3, and markers and electrodes for positioning the optical modulator 1. However, the marker is not shown for simplicity.
[0017]
The optical modulator 1, the LD element 3, the ball lens 4, and the optical isolator 5 are accurately positioned in advance so that the light emitted from the LD element 3 is coupled to the optical waveguide of the optical modulator 1 with maximum efficiency on the base 2. The processed V-groove 21 and the marker are arranged by mechanical alignment, or only the height direction and the tilt direction are aligned with mechanical accuracy, and the alignment in the horizontal plane with respect to the base 2 is an optical waveguide. This is done by monitoring the output light from the. Here, the V-groove 21 that is a recess formed in the concave plane 2 that is a low-level surface of the step can be accurately formed by forming the substrate 2 by anisotropic etching.
[0018]
Further, an LD waveguide 3 is optically connected to one of the high position surfaces of the substrate on which the steps having high position surfaces are formed on both sides of the low position surface, and the LD element 3 is optically connected to the other of the high position surfaces. Optical modulators 1 formed on the lower surface are respectively disposed. That is, the LD element 3 and the optical modulator 1 are mounted face down on the high level surface of the step of the base 2 so that the center of the ball lens 4 is positioned so that the optical axis 11 is almost close to the surface of the base 2. Take a close arrangement. At this time, a light wave propagation region is secured in the vicinity of the optical axis on the incident side. Further, the marker is also formed by the V groove 21 at the same time. With these configurations, the optical modulator 1 can be integrated with the light source with high efficiency and mounted in one housing. Further, it is possible to simplify the complicated polarization-maintaining fiber and its connection process.
[0019]
Next, particularly important points in practicing the present invention will be described.
[0020]
1) Securing the optical path between the LD element and the optical modulator A groove for securing the optical path between the LD element and the optical modulator is required in addition to the groove for mounting the ball lens 4. If formed, overetching occurs at the pattern edge portion at the intersection with the V-groove of the ball lens, the edge becomes dull, and the mounting accuracy of the ball lens cannot be ensured. In addition, when a groove is formed in the light beam direction by dicing, a groove is also formed in the lower part of the LD, and there is a problem that heat dissipation of the LD element is deteriorated.
[0021]
For this reason, a concave flat surface 22 which is a low-position surface of a step is formed on the element mounting portion and the ball lens mounting portion, the periphery of the V groove 21 is a planar structure, and the element mounting portion is located at a position several tens of μm away from the V groove 21. A method of securing an optical path by configuring the terrace 23 was used. The height of the terrace 23 varies depending on the design of the optical system to be described later, but is preferably about 60 μm, for example, so that an optical path and a necessary ball height can be secured at the same time. The terrace 23 is formed by, for example, patterning a V groove for a marker and a V groove for mounting a ball lens at the same time, producing an etching mask, forming a V groove 21 by anisotropic etching, and then forming the V groove 21. There is a method of mechanically grinding the periphery extensively.
[0022]
2) Compensation of difference in thermal expansion coefficient between LN and Si The thermal expansion coefficients α of LN and Si are about 15.4 × 10 ⁻⁶ and 2.6 × 10 ⁻⁶ , respectively, and the values are different by one digit. Therefore, a large stress is applied to the joint and it is difficult to maintain the parallelism between the substrates. Further, there is a problem that a tilt angle is generated in the incident optical system due to an environmental change, and the coupling efficiency to the optical modulator 1 varies due to the change.
[0023]
Therefore, by reducing the bonding area between LN and Si, the mechanical stress is reduced, and at the same time, the distance between the object images of the lens is reduced, thereby avoiding the influence of the tilt angle between the substrates due to the stress and the LD element. The mechanical strength of the mounting portion can be ensured. For example, good characteristics can be obtained by setting the chip width to 1.0 to 2.0 mm, the width of the joint in the optical axis direction to 1.5 to 2.0 mm, and the distance between object images to about 1 mm.
[0024]
3) Control of polarization direction The output light of the LD element 3 is TE mode linearly polarized light. However, in face-down mounting, the optical modulator 1 is limited to the operation in the TE mode waveguide. In order to sufficiently suppress the return light from the end face of the optical modulator 1 to the LD element 3, an optical isolator 5 is generally inserted between the LD element 3 and the optical modulator 1. Since the outgoing light from the light rotates 45 ° from the original polarization direction and enters the optical modulator 1, both TE and TM modes are excited. That is, it is necessary to control the polarization direction.
[0025]
For this reason, the optical isolator 5 is composed of the first polarizer 52, the Faraday rotator 51, the second polarizer 53, and the half-wave plate 54 in order from the light source side, and the polarization angle is controlled by the half-wave plate 54. The method to be used is used. In order to guide the TM mode in the Z-cut LN substrate, the TE mode light of the light source rotated 45 ° by the optical isolator 5 is further rotated 45 °, and the polarization angle is rotated 90 ° with respect to the light source. For this purpose, the optical axis (crystal optical axis) of the half-wave plate 54 is inclined by 22.5 ° from the polarization direction of the polarizer 53. Similarly, in order to guide the TE mode, the optical axis of the half-wave plate may be inclined by −22.5 ° with respect to the polarization direction of the polarizer 53. In this way, the optical isolator 5 is a half of the main surface of a polarizing plate that transmits only linearly polarized light, with the crystal optical axis rotated to the left or right by a predetermined angle with respect to the polarizing transmission axis of the polarizing plate. A polarizer to which a wave plate is bonded is formed on either the light incident side or the light emitting side of the Faraday rotator.
[0026]
FIG. 4 shows the basic operation of the optical isolator 5. In the figure, the arrow in the circle indicates the polarization direction. The outgoing light L1 from the LD element 3 passes through the first polarizer 52, rotates by 45 ° by the Faraday rotator 51, and passes through the second polarizer 53 which is previously inclined by 45 °. Next, it is further rotated by 45 ° by the half-wave plate 54, and 90 ° linearly polarized light enters the light modulator 1 with respect to the polarization direction of the light source. The return light L2 from the optical modulator 1 passes through only the −45 ° component by the second polarizer 53, rotates 45 ° by the Faraday rotator 51, and the polarized light having the same angle as the original return light is the first. Is incident on the polarizer 52. Since the first polarizer 52 has a large loss with respect to the polarization of 90 ° with respect to the polarization of the light source, the return light L2 does not pass and the return light to the LD element 3 is suppressed.
[0027]
FIG. 3 shows a second embodiment of the present invention. In the first embodiment, the LD element 3 and the optical modulator T are connected using a single lens system. However, in the optical module M2 of the present embodiment, two ball lenses 41 are provided before and after the optical isolator 5. , 42 are used to constitute a parallel optical system. Thereby, the angle dependence of the light passing through the optical isolator 5 can be suppressed. Reference numerals 21 and 24 denote V grooves formed by anisotropic etching for disposing each ball lens.
In the embodiment of the present invention, an optical modulator is described as an optical waveguide body and a light emitting element is used as an optical semiconductor element. However, the present invention is not limited to this, and various optical waveguide bodies such as an optical switch and photo The present invention can be applied to a light receiving element such as a diode, and can be appropriately changed without departing from the gist of the present invention.
[0028]
【The invention's effect】
As described above in detail, the optical module according to the present invention has the following remarkable effects.
[0029]
Both the typical optical semiconductor element (LD element) and the near field of the optical modulator have an asymmetric shape extending long in the horizontal direction of the waveguide, and the aspect ratio is, for example, about 1: 3 for the LD element. The modulator is about 1: 2. Therefore, if the optical semiconductor element and the optical modulator are directly coupled, the shape is relatively close to a fiber having a circular mode field shape.
[0030]
Therefore, the mode field matching can be easily adjusted optimally by an operation of placing it horizontally on the substrate and directly coupling it. Further, it is not necessary to insert fibers having different mode field shapes between them, and more ideal coupling is possible. Instead of the ball lens, an aspheric lens mounted on a cylindrical lens holder may be mounted on the V-groove, and in this case, the coupling efficiency can be further increased.
[0031]
-Since there is no deterioration of linearly polarized light after the optical isolator, it is not necessary to insert a polarizer such as Lamipol on the incident side of the optical modulator. This greatly improves the assembly efficiency and reliability due to the reduction in the number of parts.
[0032]
The adjustment of the polarization direction, which has been complicated in the past, becomes easy with a simple process of placing the optical semiconductor element and the optical modulator on the substrate.
[0033]
-The package of the optical semiconductor element can be shared with the optical modulator. Since the polarization-maintaining fiber that conventionally connects the optical semiconductor element and the optical modulator is not required, the number of parts can be greatly reduced. In addition, handling becomes easy and system maintenance is improved.
[0034]
-An optical module in which an optical semiconductor element and an optical modulator can be optically connected easily and with high efficiency, polarization control is easy, and an excellent optical module with high flexibility can be provided. Moreover, the number of parts can be reduced by integration, and reliability is improved. As a result, an optical module with high optical output, small size, and high reliability can be realized.
[Brief description of the drawings]
1A and 1B are diagrams illustrating an embodiment of an optical module according to the present invention, in which FIG. 1A is a plan view of the optical module, and FIG. 1B is a side view of the optical module.
FIG. 2 is a perspective view for explaining an embodiment of an optical module according to the present invention.
3A and 3B are diagrams for explaining another embodiment of the optical module according to the present invention, in which FIG. 3A is a plan view, and FIG. 3B is a cross-sectional view taken along line BB in FIG.
FIG. 4 is a diagram for explaining the operation of the optical isolator according to the present invention.
FIG. 5 is a schematic diagram showing the structure of an optical module according to the prior art.
1: Optical modulator (optical waveguide)
2,12: Base 3: LD element (optical semiconductor element)
4: Ball lens (condensing lens or collimating lens)
5: Optical isolator 6: Optical fiber 11: Optical path 21, 24: V groove 22: Low position surface of step (concave surface)
23: Terrace M1, M2: Optical module

Claims (1)

An optical semiconductor element is disposed on one of the high-position surfaces of the base on which a step having high-position surfaces is formed on both sides of the low-position surface, and an optical waveguide that is optically connected to the optical semiconductor element is disposed on the other surface of the high-position surface. Each of the optical modulators formed on the surface is disposed face down, and a condensing lens and an optical isolator optically connected to the optical semiconductor element and the optical modulator are disposed on the low position surface. formed is, the optical modulator is made is the optical waveguide for guiding the TM mode to the substrate made of a Z-cut lithium niobate is formed, the light isolator, the first polarizer from the optical semiconductor element side An optical module comprising: a Faraday rotator, a second polarizer, and a half-wave plate whose optical axis is inclined by 22.5 ° from the polarization direction of the second polarizer .

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