Optical module

Provided is an optical module capable of inhibiting both the displacement of an optical axis caused by thermal changes and property degradation in an optical functional circuit. The optical module includes: a planar lightwave circuit including a waveguide-type optical functional circuit and a waveguide region where only an optical waveguide is formed in contact with a side, wherein an emission end face where output light is emitted from the optical functional circuit, or an entrance end face where input light is entered to the optical functional circuit is formed in contact with the side; a fixing mount employed to hold the planar lightwave circuit only in the portion where the waveguide area is located; and an auxiliary mount employed to hold the planar lightwave circuit in contact with a side that is opposite the side where the emission end face or the entrance end face.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Patent Application No. 2013-002991, filed Jan. 10, 2013, which is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical module provided by integrating a planar lightwave circuit with a light-emitting element, a light-receiving element or an optical functional element.

2. Description of the Related Art

The development of optical components has become increasingly important with advances in the optical communication technology. Above all, an optical transceiver has been contemplated to increase transmission speed and response speed, thereby increasing its communication capacity. A commonly used transceiver includes a light-emitting element or a light-receiving element, formed by using an optical semiconductor, and an optical fiber for input or output, where these components are optically coupled through a lens. In an optical receiver, for example, light emitted from an optical fiber at the input side is collected to the light-receiving element through the lens, and is detected by direct detection (intensity detection).

As for a modulation/demodulation processing technique in an optical transmission system, signal transmission using a phase modulation scheme has been widely practiced. A phase shift keying (PSK) scheme is a scheme for transmitting signals by modulating the optical phase, and with this scheme, the transmission capacity has been increased exponentially by performing multilevel modulation.

In order to receive such PSK signals, detection of optical phase is required. A light-receiving element is capable of detecting the intensity of signal light, but is incapable of detecting the optical phase, and thus a method for converting the optical phase to the optical intensity is required. It is noted that a method for detecting a phase difference by employing optical interference. With this method, the signal light is interfered with another light (reference light), and the optical intensity of the interfering light is detected by a light-receiving element to obtain optical phase information. The detection method employed may be coherent detection using a light source separately provided as reference light, or differential detection for splitting signal light and employing a split portion of the light to foe interfered as reference light with the signal light. As described above, unlike the conventional optical receiver employing only an intensity modulation scheme, a recent PSK optical receiver requires an optical interferometer that converts phase information to intensity information by employing optical interference.

Such an optical interferometer can be implemented by using a planar lightwave circuit (PLC). The planar lightwave circuit has superior features in terms of mass productivity, low cost and high reliability, and various types of optical interferometers can be implemented. An optical delay line interferometer or a 90-degree hybrid circuit, for example, is provided as the optical interferometer used in the PSK optical receiver for practical use. Such a planar lightwave circuit can be formed by a standard photolithography method, an etching technique, and glass deposition techniques such as flame hydrolysis deposition (FHD).

In view of a specific forming process, first, an underclad layer formed mainly of silica glass and a core layer having a refractive index higher than that of a clad layer are deposited on a substrate, such as an Si substrate. Then, various waveguide patterns are formed on the core layer, and at the end, she waveguide formed of the core layer is embedded in an overclad layer. Through such a process, a waveguide-type optical functional circuit is obtained. The signal light is confined in the waveguide that is produced via the above process, and is propagated inside the planar lightwave circuit.

FIG. 1illustrates a method for optically connecting a conventional planar lightwave circuit to an optical receiver. In view of the method for optically connecting a planar lightwave circuit to an optical receiver in a PSK optical receiver, the basic connection between the two is a simple fiber connection, as illustrated inFIG. 1. Here, a planar lightwave circuit1where optical fibers3aand3bare connected respectively to the input and output ends is connected by optical fibers to an optical receiver2that includes an input optical fiber3b, so that optical coupling between the two is established. The number of optical fibers used for optical coupling can be determined by the number of output lights emitted from the planar lightwave circuit, and may be more than one. However, there has been a problem that when such optical fiber connection is employed, the size of the optical module is increased. To avoid this problem, the output of the planar lightwave circuit and the input of the optical receiver are optically coupled directly by using a lens to provide the whole structure as an integrated package, and as a result, the reduction of the size is enabled. The optical module wherein a planar lightwave circuit and an optical receiver are optically coupled directly is called an integrated optical receiver.

A method for fixing the planar lightwave circuit becomes critical to implement the integrated optical receiver. In a case where the light emitted by the planar lightwave circuit is to be propagated in space and to be coupled to the light-receiving element by using a lens or the like, when the positions of the light emission end, the lens and the light-receiving element are changed relative to each other, all the light may not be received by the light-receiving element, and loss of light may occur. Since the positions of those are particularly varied due to thermal expansions when the temperature of the package storing the optical receiver, the ambient temperature, or the temperature of the individual demerits, etc. changes, the above problem becomes more pronounced. Therefore, in order to perform optical coupling with low loss, the positions of these elements should not be varied at least relative to each other even when the ambient temperature, etc. is changed.

In particular, change in the shape of the planar lightwave circuit, which is caused by thermal expansion due to a change in the ambient temperature, is substantially greater that of the light-receiving element. Further, the area of the optical module that the planar lightwave circuit occupies is about one or two digits larger than the area occupied by the light-receiving element, and the shape change in the planar lightwave circuit due ho thermal expansion is also one or two digits greater than that in the light-receiving element. Furthermore, since there as a great difference in the thermal expansion coefficients between the substrate of the planar lightwave circuit and the deposited thin glass, significant warping occurs due to thermal changes. Accordingly, displacement for light emission from the planar lightwave circuit and a change in the emission angle with respect to the light-receiving element are more important. These two changes affect changes in the positions and angles of light emitted from the planar lightwave circuit, and cause displacement of an optical axis. The displacement of the optical axis degrades the performance or optical coupling relative to the light-receiving element, and causes losses in the optical coupling. For the implementation of the integrated optical receiver, it is critical that such displacement of the optical axis be resolved, or be free from adverse effect.

FIG. 2illustrates the internal structure of a conventional integrated optical module. A method for rigidly fixing almost the entire bottom surface of the planar lightwave circuit is known to prevent the occurrence of aforementioned displacement of an optical axis due to the thermal changes. In the integrated optical receiver illustrated inFIG. 2, a planar lightwave circuit13that includes an optical interferometer as an optical functional circuit, a lens14and a light-receiving element15are fixed to a base substrate11by employing, respectively, fixing mounts12a,12b, and12cthat serve as supporting members. An optical fiber16and the planar lightwave circuit13are connected through an optical fiber fixing component17. Light that has entered along the optical fiber16is interfered in the planar lightwave circuit13, and is thereafter coupled to the light-receiving element15by the lens14.

The fixing mount12aand the planar lightwave circuit13are fixed by an adhesive18or a bonding material, such as solder. Almost the entire bottom surface of the planar lightwave circuit13is rigidly fixed to the fixing mount to suppress the thermal expansion and warping changes. Further, since the lens14and the light-receiving element15are also fixed to the fixing mounts, displacement of an optical axis due no thermal changes is prevented.

The structure ofFIG. 2allows to substantially inhibit the displacement of an optical axis due to thermal changes, while change in the property of she planar lightwave circuit due to thermal changes becomes prominent. As mentioned previously, since the planar lightwave circuit13is formed of an Si substrate13aand a silica glass layer13bhaving a great difference in the thermal expansion coefficients therebetween, the change of warping and thermal expansion due to thermal changes become significant. In the structure illustrated inFIG. 2, the entire bottom surface of the planar lightwave circuit13is fixed, and therefore, thermal expansion and warping changes are limited.

Meanwhile, in such a structure, high thermal stress is generated between the Si substrate13aand the silica glass layer13b. The stress causes a refractive index change inside the silica glass layer13bthrough the photo elastic effect. For the optical interferometer formed in the planar lightwave circuit13, the length of the waveguide and the refractive index are precisely adjusted to control the interference property. The refractive index change caused by the stress changes the equivalent circuit length and also the property of the interferometer, thereby causing degradation in the property of the optical interferometer.

In this regard, when an elastic adhesive, a soft adhesive such as paste, or fixing paste is used as the adhesive18in order to suppress the occurrence of thermal stresses for limiting changes in the optical property (see, for example, Patent Literature 1), the affect of the aforementioned displacement of an optical axis may become noticeable, and a loss may occur.

Furthermore, a wavelength selective switch is known as an optical module provided by integrating a planar light wave circuit with an optical functional element (see, for example, Patent Literature 2). A planar lightwave circuit employed for a wavelength selective switch is an optical circuit wherein an arrayed waveguide optical input/output circuit that includes an input/output waveguide, a slab waveguide and an arrayed waveguide is formed. The size (the length of the long side) of the optical circuit inFIG. 2that includes an optical interferometer is about 10 mm to 20 mm, while the size (the length of the long side) of the optical circuit that includes an arrayed waveguide optical input/output circuit is large, about 30 mm to 200 mm (see, for example, Non-Patent Literature 1).

The increase in size of the planar lightwave circuit causes the increase in the change of a warp due to thermal changes and the increase in the change of the distance of extension due to thermal expansion. As another problem, reliability against vibrations end shocks, particularly, to a drop of a resonance frequency, is reduced, and the stress applied to the planar lightwave circuit by optical fiber fixing parts is increased, so that the change of the above described optical properties would be increased.

CITATION LIST

Patent Literature

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical module capable of inhibiting both the displacement of an optical axis caused by thermal changes and property degradation in an optical functional circuit.

To achieve the above object, one embodiment of the present invention is characterized by comprising: a planar lightwave circuit including a waveguide-type optical functional circuit that is formed on a substrate, and a waveguide region where only an optical waveguide is formed in contact with a side, wherein an emission end face of the optical waveguide where output light is emitted from the optical functional circuit, or an entrance end face of the optical waveguide where input light is entered to the optical functional circuit is formed in contact with the side; a fixing mount employed to hold the planar lightwave circuit only in the portion where the waveguide area is located; and an auxiliary mount employed to hold the planar lightwave circuit in contact with a side that is opposite the side in contact with the emission end face or the entrance end face, wherein the planar lightwave circuit and the auxiliary mount are fixed by employing an elastic adhesive or an elastic structure that has lower elasticity than that of an adhesive or a bonding material that rigidly fixes the planar lightwave circuit to the fixing mount.

As described above, according to the present invention, since the planar lightwave circuit is fixed by employing only the waveguide region where only the optical waveguide is formed, the position of the emission end face or the entrance end face of the waveguide region can be fixed, without being affected by warping change of the planar lightwave circuit due to the thermal change, or by the horizontal positional change due to thermal expansion. Further, since the region for the optical interferometer of the planar lightwave circuit is not fixed, the properties can be stabilized.

DESCRIPTION OF THE EMBODIMENTS

In the embodiments, the waveguide region for input/output of light within an optical circuit prepared on a planar lightwave circuit, that should be especially rigidly fixed to prevent displacement of an optical axis, is only fixed to a fixing mount. The region where an optical functional circuit, such as an optical interferometer that is susceptible to stress, is formed is not fixed to the fixing mount. This allows to minimize the effect of stresses at the optical functional circuit even when distortion or warping has occurred due to thermal changes, and therefore, the degradation of the property of the optical functional circuit can be inhibited. Further, since the waveguide region is fixed to the fixing mount, the displacement of an optical axis caused by thermal changes can be inhibited, and an operating margin for an optical module with respect to thermal changes can be increased.

Furthermore, the planar lightwave circuit is fixed to an auxiliary mount by employing the region opposite to the region that is fixed to the fixing mount. At this time, when an elastic adhesive is employed for fixing, not only variations of expansion or warping caused by the temperature can be absorbed, but also vibrations generated by the planar lightwave circuit can be reduced, and further, the force applied by optical fiber fixing components can be absorbed.

FIG. 3illustrates the internal structure of an optical module in accordance with Embodiment 1 of the present invention. A planar lightwave circuit33where an optical interferometer is formed as an optical functional circuit, a lens34and a light-receiving element35are respectively fixed to a base substrate31by fixing mounts32a,32b, and32cthat serve as supporting members. An optical fiber38is connected to the planar lightwave circuit33through an optical fiber fixing component37. In an integrated optical receiver, when light has entered from the optical fiber36, the optical signal processing, such as interference, is performed for the light by the planar lightwave circuit33, and thereafter, the obtained light is coupled to the light-receiving element35through the lens34. The planar lightwave circuit33is provided by laminating, on an Si substrate33a, a silica glass layer33bwhere a waveguide-type optical functional circuit formed of a core layer and a clad layer is provided.

FIGS. 4A to 4Dillustrate a method for fixing the planar lightwave circuit in accordance with Embodiment 1. The method for fixing the planar lightwave circuit33illustrated inFIG. 3will now be described in detail. As shown inFIG. 4A, the silica glass layer33bof the planar lightwave circuit33includes a region33ywhere an optical interferometer is formed as an optical functional circuit, and a waveguide region33xwhere only an optical waveguide is formed (an optical interferometer is not formed) is contact with one side. An emission end face of the optical waveguide where output light is emitted to the lens34is formed in contact with the one side. The fixing mount32ahas an inverted L-shape or a hook shape in a side view as illustrated inFIG. 3, and the waveguide region33xwithin the optical circuit prepared on the planar lightwave circuit33is only fixed by employing an adhesive38or a bonding material, such as solder (seeFIG. 4B).

The region33yof the optical interferometer of the planar lightwave circuit33is not fixed to the fixing mount32a, and is held above the fixing mount32a. The shape of the planar lightwave circuit33can be changed, as desired, regardless of the occurrence of warping due to thermal changes (seeFIGS. 4C and 4D), and is insusceptible to stresses. This allows to secure the position of the emission end of the waveguide region33x, without being affected by any warping changes of the planar lightwave circuit33due to thermal changes and without any positional changes in a horizontal direction (with respect to the circuit plane of the planar lightwave circuit33) due to thermal expansion. Since the lens34and the light-receiving element35are also fixed to the fixing mount, displacement of an optical axis due to thermal changes does not occur.

The stress or stress changes caused by implementation induces birefringence changes in the optical waveguide. Since the optical interferometer is sensitive to the birefringence changes and is susceptible to property degradation, the property can be stabilized by not fixing the region33yof the optical interferometer to the fixing mount32a. The waveguide region33x, on the other hand, is fixed to the fixing mount32a, however, the property degradation due to the birefringence changes can be reduced, because the planar lightwave circuit33has a small effect of stresses due to warping changes, compared to a case wherein the entire surface of the planar lightwave circuit is fixed to the fixing mount.

A difference in height relative to the fixed part should be provided for the part of the fixing mount32awhere the planar lightwave circuit33is not fixed (unfixed part), so that the planar lightwave circuit33and the upper surface of the unfixed part do not contact each other even when warping change of the planar lightwave circuit33is caused due to thermal changes. Otherwise, if the planar lightwave circuit33is in contact with the fixing mount32aas a result of a warping change, stress to the substrate will be generated, and this will lead to property degradation. In a case wherein the planar lightwave circuit33is formed of an Si substrate and a silica-based glass material, a height difference h of approximately several hundred μm should be provided between the unfixed part and the fixed part (seeFIG. 4C).

The description has been given for a case wherein the planar lightwave circuit33is formed of an Si substrate and a silica-based glass material, however, the planar lightwave circuit33may be entirely formed of a semiconductor material or a glass-based material, or may be formed of a dielectric material, such as LiNbO3. In either case, the effect of stresses caused by implementation of the planar lightwave circuit and by thermal changes can be suppressed.

The planar lightwave circuit and the fixed part of the fixing mount can foe further limited to prevent degradation of properties upon thermal changes or at the time of implementation. Specifically, as illustrated inFIG. 5A, a fixing mount42awith a fixed part having a limited shape is employed, instead of the fixing mount32aillustrated inFIG. 4. As illustrated inFIG. 5B, the waveguide region33zin the planar lightwave circuit33is limited to a portion along the side of the emission end face in consonance with the shape of the fixed part of the fixing mount42a, so that the positron of the emission end of the waveguide region33zcan be fixed without any positional changes in horizontal direction doe to thermal expansion. Compared with the structure shown inFIG. 4, this allows to further reduce stresses generated by implementation and stress changes caused by thermal changes.

So long as the planar lightwave circuit and the fixed part of the mounting mount are located near the light emission end, except for the region33y, the same effects can be obtained. Therefore, the area for the fixed part is not limited to the lower face of the Si substrate33aof the planar lightwave circuit33, and may be the upper face of the silica glass layer33bor the end face of the emission end face.

FIG. 6illustrates the internal structure of an optical module in accordance with Embodiment 2 of the present invention. A planar lightwave circuit53where an arrayed waveguide optical input/output circuit is formed as an optical functional circuit, a lens54and an optical functional element55are respectively fixed to a case substrate31by fixing mounts52a,52b, and52cthat serve as supporting members. An optical fiber56is connected to the planar lightwave circuit53through an optical fiber fixing component57. In a wavelength selective switch, when light has entered from the optical fiber56, the optical signal processing, such as optical combining/splitting or optical shaping to adjust the diameter of a beam, is performed for the light by the planar lightwave circuit53, and thereafter, the obtained light is coupled to the optical functional element15through the lens54. The planar lightwave circuit53is provided by laminating, on an Si substrate53a, a silica glass layer53bwhere a waveguide-type optical functional circuit formed of a core layer and a clad layer is provided.

A method for fixing the planar lightwave circuit53will now be described. As well as in Embodiment 1, the waveguide region within the optical circuit prepared on the planar lightwave circuit53is only fixed to the fixing mount52aby employing an adhesive58. As shown inFIG. 4, the fixing mount52amay be fixed along one side where the emission end face of the optical waveguide is formed for propagating light to be emitted to the lens54, or as shown inFIG. 5, a limited shape may be employed for the fixed part. A commonly adhesive, such as acrylic adhesive or epoxy-based adhesive, may be employed as the adhesive58, and the elasticity after the adhesive has cured, of about 10 MPa or more is appropriate. The planar lightwave circuit is rigidly fixed by employing this adhesive58to prevent the optical coupling of the planar lightwave circuit53to the lens54and the optical functional element55from the affect due to the thermal changes. Further, when rigid fixing is available, the living process may be performed not only by employing the adhesive, but also by employing a bonding agent, such as solder, or by welding. So long as the planar lightwave circuit53can be rigidly fixed by fixing mount52a, and in a case wherein, as shown inFIG. 5, the shape of the fixed part is limited, the fixing mount52aof a 3 to 10 mm square is employed for a substrate of 400×100 mm.

Meanwhile, an auxiliary mount61is provided for the base substrate51to support the planar lightwave circuit53in contact with the side opposite the side along which the emission and face of the planar lightwave circuit is formed. Here, the planar lightwave circuit53and the auxiliary mount61are fixed together by employing an elastic adhesive62. The elastic adhesive62has the elasticity after the adhesive has cured, of about 0.1 MPa or smaller, which is lower than that of the adhesive58. An example well known adhesive of this type is a modified silicone-based adhesive. Further, the thickness of the adhesive58layer is 5 to 20 μm, while the thickness of the elastic adhesive62layer is about 100 μm to 1 mm for absorbing the expansion and warping change due to the temperature.

Further, when instead of the elastic adhesive, an elastic structure, such as a spring that has about the same elastic modulus, is employed for fixing, the same effects can be obtained.

The long side of the planar lightwave circuit53where the arrayed waveguide optical input/output circuit is formed is long, about 30 mm to 200 mm, and therefore, if only the fixing mount is employed for fixing the planar lightwave circuit53in the same manner as in Embodiment 1, the resonant frequency of the planar lightwave circuit53would foe reduced (2 kHz or lower), and the reliability relative to the vibrations will be deteriorated. Further, in a case wherein the optical fiber56connected via the optical fiber fixing part57is a fiber array having a plurality of cores, the force applied to the planar lightwave circuit53by these members would be increased. In some cases, while taking the strength into account, it is difficult that these members are supported at the location apart from the fixing mount52, i.e., along the side opposite the side where the emission end face is formed.

In Embodiment 2, since the planar lightwave circuit53is fixed to the auxiliary mount61, by using the elastic adhesive62, along the side opposite the side where the emission end face is formed, the vibrations of the planar lightwave circuit53can be reduced, and the force applied by the optical fiber fixing part57can be absorbed. Even in a case wherein there is a difference in the thermal expansion coefficients between the planar lightwave circuit53and the base substrate51, a difference of the distance of extension due to thermal expansion can be absorbed by using the elastic adhesive62. Therefore, the auxiliary mount61is required tor absorption of vibrations and stress, and one or two fixing mounts52aof about 5 mm square are employed for a substrate of 40×100 mm, for example.

FIG. 7illustrates the internal structure of an optical module in accordance with. Embodiment 3 of the present invention. A planar lightwave circuit53where an optical combining/splitting circuit is formed as an optical functional circuit, a lens54and an optical functional element55are respectively fixed so a bass substrate51by fixing mounts52a,52b, and52cthat serve as supporting members. An optical fiber56is connected to the planar lightwave circuit53through an optical fiber fixing component57. As well as in Embodiment 2, the waveguide region within the optical circuit prepared on the planar lightwave circuit53is only fixed to the fixing mount52aby employing an adhesive58. Meanwhile, the planar lightwave circuit53and an auxiliary mount61are connected together via an intermediate auxiliary mount63.

For the procedures for fixing the planar lightwave circuit53, first, the planar lightwave circuit53is fixed to the fixing mount52ato establish an optical connection between the planar lightwave circuit53and the optical system. Then, the planar lightwave circuit53is connected to the auxiliary mount61. However, for the structure shown in Embodiment 2, when an elastic adhesive62is applied between the planar lightwave circuit53and the auxiliary mount61, there is a possibility that the adhesive62might shrink while curing, and apply stress to the planar lightwave circuit53. Further, there is also a possibility that the elastic adhesive may stretch by shrinkage during curing, and fail in the state that there is no flexibility left to absorb vibration or stress.

Therefore, in Embodiment 3, the planar lightwave circuit53is fixed to the intermediate auxiliary mount63by employing the elastic adhesive62, and thereafter, is fixed to the fixing mount52ato establish an optical connection between the planar lightwave circuit53and the optical system. Next, the intermediate auxiliary mount63is fixed to the auxiliary mount61by using an adhesive64. As a result, applying of unnecessary stress to the planar lightwave circuit33by the elastic adhesive62is prevented.

The method for fixing the intermediate auxiliary mount63to the auxiliary mount61is not limited to the use of an adhesive, and a method, such as fastening by screws or welding, may also be employed to obtain the effects to prevent the application of unnecessary force.

FIG. 8illustrates the internal structure of an optical module in accordance with Embodiment 4 of the present invention. A planar lightwave circuit53where an optical combining/splitting circuit is formed as an optical functional circuit, a lens54and an optical functional element55are respectively fixed to a base substrate51by fixing mounts52a,52b, and52cthat serve as supporting members. A connection to an optical fiber is not shown for simplifying the drawing. As well as in Embodiment 2, the waveguide region within the optical circuit prepared on the planar lightwave circuit53is only fixed to the fixing mount52aby employing an adhesive58. Meanwhile, an auxiliary mount65has a C-shape in a side view as shown inFIG. 8, and the planar lightwave circuit53is fixed to the auxiliary mount65by using two portions, i.e., the face of an Si substrate53aand the face of a silica glass layer53b.

In Embodiments 2 and 3, there is a possibility that, when the elastic adhesive62is thermally expanded or shrunk due to the change of the ambient temperature where the optical module is employed, a stress may be applied to the planar lightwave circuit53. Therefore, in Embodiment 4, when the planar lightwave circuit53is to be fixed to the auxiliary mount65, the same amounts of elastic adhesives62aand62bare applied substantially to the face of the Si substrate53aand the face of the silica glass layer53b, respectively, and are cured at the same time. As a result, almost the same stress is applied to the face of the Si substrate53aand to the face of the silica glass layer53b, and the stresses to be exerted to the planar lightwave circuit53are offset each other. Further, since the elastic adhesives62aand62bare cured at the same time, the stresses due to the shrinkage of the elastic adhesives during curing can also be offset, as described in Embodiment 3.

It should be noted that a gap between the planar lightwave circuit53and the auxiliary mount65is about 100 μm to 1 mm, as described above. Furthermore, the portion on the plane where the auxiliary mount65in the C-shape and the planar lightwave circuit53overlap each other, i.e., the area where the elastic adhesive62aor62bis applied is a size of a 5 mm square. Therefore, when the surface tension is employed for applying the elastic adhesive that is not yet cured, almost the same amount of adhesive can be applied. In principle, even when the thicknesses of the elastic adhesive62alayer and the thickness of the elastic adhesive62blayer differ from each other, this does not affect the balance of upward and downward stresses, and therefore, there is an advantage that high mechanical accuracy is not required for incorporating thy planar lightwave circuit53in the optical module.

FIG. 9illustrates the internal structure of an optical module in accordance with Embodiment 5 of the present invention. A planar lightwave circuit53where an optical multiplexing/demultiplexing circuit is formed as an optical functional circuit, a lens54and an optical functional element55are respectively fixed to a base substrate51by fixing mounts52a,52b, and52cthat serve as supporting members. A connection to an optical fiber is not shown for simplifying the drawing. As well as in Embodiment 2, the waveguide region within the optical circuit prepared on the planar lightwave circuit53is only fixed to the fixing mount52aby employing an adhesive58. Meanwhile, an auxiliary mount66has a C-shape in a side view. The planar lightwave circuit53is fixed to the auxiliary mount66by using two portions, i.e., the face of an Si substrate53aand the face of a silica glass layer53b, and the area size for fixing the face flower face) of the Si substrate53ais greater than the area size for fixing the face (upper face) of the silica glass layer53b.

Since the planar lightwave circuit33has a layer structure using various different types of materials, there is a possibility that warp may be changed due to thermal changes. In a case wherein the temperature dependence of warp is satisfactorily low, the structure described in Embodiment 4 is appropriate. However, in a case wherein the temperature dependence of warp is high, the planar lightwave circuit53is restricted by the auxiliary mount, and therefore, new stress is generated due to thermal changes.

Therefore, in Embodiment 5, different bonding area sizes are employed for the upper face and the lower face, so that the layer thicknesses of the elastic adhesive62aand62bthat have been displaced (shrunk and expanded, respectively) by the occurrence of the warp of the planar lightwave circuit53due to thermal change can match the layer thicknesses of the elastic adhesives62aand62bobtained when the forces exerted by the elastic adhesives are equaled.

FIGS. 10A to 10Cillustrate the internal structure of an optical module in accordance with Embodiment 6 of the present invention. In Embodiments 1 to 5, the planar lightwave circuit53is horizontally mounted on the base substrate51, while in Embodiment 6, a planar lightwave circuit is mounted perpendicularly to a base substrate51. As well as Embodiments 1 to 5, a planar lightwave circuit53, lens54and an optical functional element55are included as an optical functional circuit, and are fixed to the base substrate51by employing fixing mounts52a,52band52c. An optical fiber56is connected to the planar lightwave circuit53via an optical fiber fixing part57. The optical fiber56is a tape-fiber array having a plurality of cores.

As well as in Embodiments 1 to 5, of the optical circuit of the planar lightwave circuit53, only the waveguide region is fixed to the fixing mount52aby employing an adhesive58. Meanwhile, auxiliary mounts67aand62bhave a C-shape in a side view inFIG. 10C, and the planar lightwave circuit53is fixed to the auxiliary mounts67aand67bby using two portions, i.e., the face of an Si substrate53aand the face of a silica glass layer53b.

In Embodiment 6, the planar lightwave circuit53is fixed to the auxiliary mount67aby using an elastic adhesive62along the side opposite the side where the emission end face of the planar lightwave circuit53is formed, so that vibration of the planar lightwave circuit53can be reduced, and the force applied by the optical fiber fixing part57can be absorbed. Further, the planar lightwave circuit53is also fixed to the auxiliary mount67balong the area between the side where the emission end face of the planar lightwave circuit53is formed and the opposed side, and therefore, when the planar lightwave circuit53vibrates, the portion having a great amplitude can be held down.