Optical waveguide and manufacturing method of optical waveguide

An optical waveguide of the present invention is an optical waveguide having a first rib and a second rib being provided on a slab layer along one direction from one side to the other side and a barrier layer being connected between said first rib and said second rib, in which: the first rib includes a first taper part having a width widening from a first end in said one side to a second end connected with said barrier layer; and the second rib includes a first layer and a second layer laminated on a face of said slab layer in turn.

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2014-019229 filed on Feb. 4, 2014, the disclosure of which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

The present invention relates to an optical waveguide and a manufacturing method of an optical waveguide, and, more particularly, to an optical waveguide and a manufacturing method of an optical waveguide which reduce transmission loss of guided light.

BACKGROUND ART

As one of silicon optical waveguides, there is a silicon rib type optical waveguide of non-patent document 1 (“High extinction ratio optical switching independently of temperature with silicon photonic 1×8 switch”, Nakamura et al., OFC2012, OTu2I.3), which makes the confinement in the lateral direction strong by providing a projection shape of silicon of approximately 1 μm width to a waveguide of a silicon thin film called a slab formed onto a substrate. This rib type optical waveguide can suppress formation of higher-order modes in the waveguide, and can suppress PDL (Polarization Dependent Loss).

The size of the principal mode of light of this rib type optical waveguide is about 1 μm, while the size of the principal mode of light of a usual single mode fiber is large and is about 9 μm. When a single mode fiber and a rib type optical waveguide are connected together, a coupling loss is large because of such difference between the sizes of principal modes of light.

As one of methods to reduce such coupling loss, there is introduction of a spot-size converter of a core expansion type that makes the diameter of the principal mode of light of a rib type optical waveguide large by gradually enlarging a core toward an optical fiber. A plurality of spot-size converters have been proposed up to now, and, for example, there has been proposed, as disclosed in patent documents 1-5 (Japanese Patent Application Laid-Open No. 2001-033642, Japanese Patent Publication No. 4719259, Published Japanese translation of PCT application No. 2001-510589, U.S. Pat. No. 7,088,890 and International Publication No. WO 2012/04279), an optical waveguide having a spot size conversion function made by stacking two or more silicon core layers whose widths widen gradually.

Methods to create an optical waveguide having such multi-stage spot size conversion function include a method to form an optical waveguide having a spot size conversion function by: making an additional core layer regrow thick over a wide area including an area for forming the spot size conversion function; and applying etching after that.

However, as shown inFIG. 18, in this manufacturing method, a barrier layer903which extends in the direction crossing a rib type optical waveguide at right angles occurs between the region of the optical waveguide in the side of an optical function device and the region of the spot size conversion function in which the core layer have been made grow. The reason of this is that the region of the optical waveguide in the device side and the region of the spot size conversion function are formed separately. Specifically, it is caused by forming a mask with a margin in the boundary area between the two above-mentioned regions formed separately because one of the regions is formed after the other region is covered by a mask. When the barrier layer903exists, it is concerned that coupling to higher-order modes and transmission loss by reflection is caused. Regarding such transmission loss, it is known that the transmission loss can be suppressed by introducing taper structures901-1and901-2disclosed in non-patent document 2 (“Taper-Integrated Multimode-Interference Based Waveguide Crossing Design”, Chyong-Hua Chen, IEEE Journal of Quantum Electronics, VOL. 46, NO. 11 and pp 1656-1661) into optical waveguides before and behind the barrier layer as shown inFIG. 18.

SUMMARY

An object of the present invention is to make an optical waveguide short.

Solution to Problem

An optical waveguide to an exemplary aspect of the invention includes a first rib and a second rib being provided on a slab layer along one direction from one side to the other side, and a barrier layer being connected between the first rib and the second rib, comprising:

the first rib including a first taper part having a width widening from a first end in the one side to a second end connected with the barrier layer;

the second rib including a first layer and a second layer laminated on a face of the slab layer in turn;

the first layer having an approximately same width as the second end and being of a shape having one of a same width and a width widening from a third end being connected with the barrier layer to a fourth end in the other side;

the second layer having a second taper part having a width widening from a fifth end in the one side to a sixth end; and

both of one end and the other end of the barrier layer in the one direction having a width wider than the second end and the third end.

EXEMPLARY EMBODIMENT

Hereinafter, exemplary embodiments of the present invention will be described. Each exemplary embodiment is just illustration, and the present invention is not limited to each of the exemplary embodiments.

The first exemplary embodiment of the present invention will be described.FIG. 1indicates a bird's eye view of an exemplary configuration of an optical waveguide in the first exemplary embodiment of the present invention.

Hereinafter, an example of a structure of an optical waveguide in this exemplary embodiment will be described with reference toFIG. 1. As shown inFIG. 1, an optical waveguide100has a slab layer1, a first rib10, a second rib20and a barrier layer30.

The optical waveguide100is an optical waveguide in which the first rib10and the second rib20are provided on the slab layer1along one direction from one side to the other side, and, further, the barrier layer30is joined between the first rib10and the second rib20. InFIG. 1, “one side” is the front left side of the illustration and “the other side” is the right back side of the illustration. Also, as shown inFIG. 1, the thickness direction of the barrier layer30is a direction of the thickness of the barrier layer in one direction from “one side” to “the other side”.

The first rib10has a first taper part13, the width of which is made wider from a first end11in “one side” toward a second end12connected to the barrier layer30. Thus, by the width of the first rib10changing gradually and continuously, the shape of the principal mode of guided light can be converted toward the width direction ofFIG. 1with low loss.

The second rib20has a first layer21and a second layer41laminated on a face of the slab layer1in turn.

The first layer21is the same width as the second end12approximately and, in addition, is of a shape having a width that is the same or becomes wider from a third end22connected with the barrier layer30to a fourth end23in “the other side” as shown inFIG. 2. Both of one end and the other end of the barrier layer30in the above-mentioned one direction have a width wider than the second end12and the third end22. When the first layer21has a shape that becomes wider from the third end22to the fourth end23as shown inFIG. 2, the width of the first layer21changes gradually and continuously. Therefore, the shape of the principal mode of guided light can be converted toward the width direction inFIG. 2with low loss as is the case with the first rib10. In this case, the principal mode of light guided from the first end11to the fourth end23except for the barrier layer30can be converted with low loss toward the width direction continuously. Also, by making the third end22be approximately the same width with the second end12, electromagnetic field distribution of light in boundary surfaces between the second end12, the barrier layer30and the third end22is rarely different, and, as a result, coupling loss can be suppressed. Further, polarization dependence of transmission loss of guided light which occurs in the barrier layer30can be reduced by making the third end22have approximately the same width with the second end12.

The second layer41has a second taper part44that becomes wider from a fifth end42in “one side” to a sixth end43. Thus, the shape of the principal mode of guided light in the height direction inFIG. 1can be converted with low loss by the width of the second layer41changing continuously and gradually.

[Description of Manufacturing Method]

Next, a manufacturing method of the optical waveguide100of this exemplary embodiment will be described taking formation of an optical waveguide by etching as an example with reference toFIG. 3. The manufacturing method will be described as five processes indicated by A-E inFIG. 3.

A substrate300in which a core layer200is arranged on a cladding layer201is prepared (A inFIG. 3: (Preparation of substrate). The thickness of the core layer200can be made to be the height of the optical waveguide of an optical functional device to be connected, for example. As a result, etching of the upper face part of the optical waveguide of an optical functional device is unnecessary, and manufacturing becomes easy. Hereinafter, illustration of the cladding layer201will be omitted.

Next, the first rib10is formed in the core layer200(FIG. 3B: Formation of the first rib). The upper surface of the core layer200that has high flatness preliminarily can be used as an upper surface of the first rib10. In this case, there is no deterioration of the flatness by etching, and the first rib10will be an optical waveguide which is low-loss. On this occasion, the thickness of the slab layer1is made to be the same as that of the slab layer of the optical waveguide of an optical functional device. Further, the level of the upper surface of the first rib10in the height direction can be made to be the same as that of the upper surface of the barrier layer30. Both of these arrangements enable to manufacture a rib simultaneously with an optical functional device and the like, and thus manufacturing is simplified. At this stage, it is a state that an upper surface of the core layer200in an area where the second rib20is formed is left just as it is.

Next, a protection layer203which covers the first rib10is formed. This protection layer203also covers an area of the core layer200not including the first rib10, the area neighboring the first rib10. The neighboring area here means a margin that can be set up in the formation area of the protection layer203considering a formation error of the protection layer203and also aiming at covering the first rib10completely. In other words, the protection layer203partially covers an area for forming the second rib20also, and the region which is not covered by the protection layer203is made to be only the area for forming the second rib20(FIG. 3C: Formation of protection layer).

Next, an additional core layer204is formed in the area for forming the second rib20on the core layer200(FIG. 3D: Formation of additional core layer). In this process, only an area for forming the second rib20in which the core layer200is remaining just as it is being selected as the area in which the additional core layer204is made to be formed, and, further, that area has an enough size and can make the additional core layer204grow to have a good thick film.

The additional core layer204that has been formed in the process indicated inFIG. 3Dis etched in a multiple-stage manner to form the second rib20having the first layer21and the second layer41(FIG. 3E: Formation of second rib). Because the first rib10and the second rib20are etched in separated processes, the barrier layer30deriving from formation of the protection layer203is formed in an accompanying manner as shown inFIG. 3E. A detailed specific example of the process shown inFIG. 3Eis shown inFIG. 16. In the process ofFIG. 16(E-1), a protection film205is formed by lithography on the upper surface of the additional core layer204in the laminating direction, and, henceforth, the second layer41and the first layer21are formed by etching the additional core layer204and the core layer200in a multiple-stage manner from (E-2) to (E-3).

Although not shown, a cladding layer is deposited after the process ofFIG. 3Ein this manufacturing method. By arranging a cladding layer also in the upper layer, oxidation of the core layer200can be prevented and low-loss conversion of the principal mode of light can be provided for a long time. Besides the illustrated manufacturing method, other manufacturing methods which can produce the optical waveguide100of this exemplary embodiment may be used.

Next, action of the optical waveguide100will be described with reference toFIG. 1. Here, description will be made by an example in which the diameter of the principal mode of light is expanded by light being guided by the optical waveguide100in one direction from “one side” to “the other side”.

First, the diameter of the principal mode of light which enters the first rib10from the first end11side is magnified in the width direction ofFIG. 1with low loss in the course of being guided in the first taper part13. In the boundary of the second end12and the barrier layer30and the boundary of the barrier layer30and the third end22, a difference in electromagnetic field distribution of the principal mode of guided light occurs, causing transmission loss. However, because the third end22has approximately the same width with the second end12, electromagnetic field distribution of the principal mode of light at each boundary surface does not differ, and transmission loss is reduced. Further, even when light is guided in any one of a direction to expand the diameter of the principal mode and a direction to reduce it, transmission loss is the same. In addition, as a result of making the width of the third end22wide sufficiently relative to the thickness of the barrier layer30, the polarization dependence of transmission loss of guided light can be reduced.

Next, when light which enters the first layer21of the second rib20from the third end22is guided from the laminated part of the first layer21and the second layer41toward “the other side”, as the second taper part44becomes wider, the diameter of the principal mode is magnified with low loss in the laminating direction (height direction inFIG. 1) this time. Here, because the fifth end42has a limited width, the shapes of a mode of light differ significantly between before and behind its tip position generally, and thus transmission loss occurs. However, by making the third end22and the second end12approximately the same width, the width of the first layer21becomes sufficiently wider than that of the fifth end around the fifth end42. For this reason, light is shut in the first layer21much, and the electromagnetic field distribution of light is also shut inside the first layer21mostly. Therefore, few changes in electromagnetic field distribution of light occur between before and behind the tip position of the fifth end42, and thus transmission loss can be suppressed.

From the above reason, when the principal mode of light is wanted to be expanded in the height direction, it is necessary to provide a taper part in the upper stage after widening the rib width of the lower layer sufficiently in advance. Moreover, as shown inFIG. 18, in the technology described in non-patent document 1, a rib width which becomes narrow once due to taper structures901-1and901-2existing before and behind the barrier layer30is needed to be expanded once again, and thus there is a problem that the optical waveguide of the lower layer becomes long. However, because the taper structure901-2inFIG. 18is not needed if it is a structure in which a rib width is wide sufficiently just like the third end22in this exemplary embodiment, expansion of a rib width mentioned above is unnecessary, resulting in enabling suppression of the length of the optical waveguide100. By suppressing the above-mentioned length, loss caused by roughness of a side wall of an optical waveguide formed at the time of manufacturing can also be reduced.

Thus, by light being guided in the optical waveguide100in one direction from “one side” to “the other side”, the diameter of the principal mode of the light is expanded. Here, although an example in which the diameter of the principal mode of light is expanded by light being guided in the optical waveguide100in one direction from “one side” to “the other side” has been shown, it goes without saying that the diameter of the principal mode of light is reduced by light being guided in the optical waveguide100in a direction from “the other side” to “one side”.

As mentioned above, the optical waveguide100in this exemplary embodiment is an optical waveguide in which the first rib10and the second rib20are installed on the slab layer1along one direction from “one side” to “the other side”, and a barrier layer30is connected between the first rib10and the second rib20, wherein the first rib10has the first taper part13that becomes wider from the first end11in “one side” to the second end12connected with the barrier layer30; wherein the second rib20has the first layer21and the second layer41laminated on a face of the slab layer1in turn; wherein the first layer21has approximately the same width with the second end12and has a shape with a width that is the same or becomes wider from the third end22connected with the barrier layer30to the fourth end23in “the other side”; wherein the second layer21has the second taper part44with a width that becomes wider from the fifth end42in “one side” to the sixth end43; and wherein both of one end and the other end of the barrier layer30in the one direction has a width wider than the second end12and the third end22. Consequently, according to the first exemplary embodiment, it is possible to make an optical waveguide having a spot size conversion function short. Besides, the number of optical waveguides which can be produced from one substrate becomes large, and the production cost of an integrated optical functional device can be reduced.

It is desired to use silicon of a refractive index of about 3.5 for the slab layer1, the first rib10and the second rib20. That is, it is desired to use the above-mentioned silicon for the core layer200in its manufacturing process. Also, it is desired to use silicon dioxide of a refractive index of 1.5 for a cladding layer.

It is desired to make the height of the first layer21be the same as the height of the first rib10. By making the height of the second end12and the third end22be the same as described above in addition to the widths of them being approximately the same, transmission loss before and behind the barrier layer30is further suppressed.

It is desired that the end faces of the fourth end23and the sixth end43are aligned. Also, it is desired that the sixth end43has approximately the same width as the fourth end23.

It is desired that the fifth end42is near the barrier layer30as close as possible along the one direction. By this, the length of the optical waveguide100can be suppressed.

It is desirable that the sixth end43is formed aligned with the fourth end23.

Although the second rib20has total of two layers of the first layer21and the second layer41in this exemplary embodiment, more layers can be provided. The first layer21and the second layer41may be connected optically sandwiching an adhesive material or the like, or may be laminated directly.

The second exemplary embodiment of the present invention will be described.FIG. 4indicates a bird's eye view of an exemplary configuration of an optical waveguide in the second exemplary embodiment of the present invention. An optical waveguide101in the second exemplary embodiment is different from the optical waveguide100in the first exemplary embodiment in a point that the fifth end42touches a step existing in the upper surface of the barrier layer30in the height direction. That is, between the first exemplary embodiment and the second exemplary embodiment, it is different in the positions of the fifth end42along one direction from “one side” to “the other side”.

[Description of Manufacturing Method]

InFIG. 5, there are shown a process of formation of a protection layer ofFIG. 3C(the lower right inFIG. 5), the structure of the optical waveguide101ofFIG. 4(the left inFIG. 5) and a figure in which a side view and a sectional view of the optical waveguide101are compared (the upper right inFIG. 5). The protection layer203of the first rib10formed in the process ofFIG. 3Bis indicated at the lower right inFIG. 5. This protection layer203covers also part of an area for forming the second rib20up to an edge50indicated at the lower right inFIG. 5. In this state, only in an area for forming the second rib20where the core layer200is remaining just as it is, formation of the additional core layer204and multiple-stage etching is performed by a process at least different from the process for formation of the first rib10. A detailed specific example of the process shown inFIG. 3Eis shown inFIG. 17. Only the protection film205formed in the upper surface of the additional core204in a laminating direction is different from the specific example indicated in the first exemplary embodiment (FIG. 16). Thus, the first rib10and the second rib20are formed in areas before and behind the edge50taking it as a boundary along the one direction.

Because, in the process of forming the second rib20, it is not necessary to make a condition of etching be the same as that of at the time of forming the first rib10, it can be performed by a different condition. In other words, the second rib20is formed by a different process and, in some cases, by different conditions. Therefore, by a reason of different conditions, or by a manufacturing error even in a case of a process (etching in the case of this specific example) under same conditions, steps occur in the edge50and in a surface boundary51vertically to the one direction as shown inFIG. 5. The fifth end42is in contact with the step of the edge50between the steps. Although an example of each step is shown in the side view in the upper right inFIG. 5, the sizes and the like of the steps have been scale-adjusted appropriately to make it easy to be understood. As shown in the side view in the upper right inFIG. 5, the step of the edge50is caused at the time of formation of the second layer41when etching the additional core layer204from its upper part, and the step of the surface boundary51at the time of formation of the first layer21and the slab layer1.

Similarly, even when producing the structure of the optical waveguide100in the first exemplary embodiment by a process shown inFIG. 3, steps occur in the edge50and a surface boundary52before and behind the barrier layer30as shown in the side view and the sectional view along the waveguide route of light shown in the right inFIG. 6. However, as shown in the sectional view in the right inFIG. 6, it is different from the optical waveguide101in the second exemplary embodiment in a point that it has a step starting from the edge50and reaching the fifth end42(the first step53in the right inFIG. 6).

A calculation result of transmission loss caused by mismatch of modes of light due to the above-mentioned steps in the case where the widths of the first rib10and the first layer21is made to be 4 μm and the thicknesses of the barrier layer30be 3 μm is shown inFIG. 7. Here, transmission loss is the negative of a value calculated by subtracting an emission light intensity from an incident light intensity of guided light. The left ofFIG. 7indicates a result when the fifth end42is of a structure of the optical waveguide100. The right ofFIG. 7indicates a result when the fifth end42is of a structure of the optical waveguide101in the second exemplary embodiment. The minus side of the horizontal axis in the figure indicates a case where a step in the side of the first rib10is lower, and the plus side a step in the side of the first rib10is higher.

As shown inFIG. 7, when a step or a difference in height is formed between the first rib10and the second rib20, loss is caused. As indicated in the left inFIG. 7, loss in the minus side is larger in the horizontal axis (etching height difference). As indicated in this exemplary embodiment, loss in the minus side can be suppressed by making it be of a structure in which the fifth end42touches the step of the edge50which is in the upper surface of the barrier layer30in the height direction as indicated in the right inFIG. 7. For example, as indicated in the right inFIG. 7, loss improvement of 0.005 dB can be realized against 0.2 μm of difference, for example, compared with the structure of the optical waveguide100of the first exemplary embodiment in the left inFIG. 7. Thus, the optical waveguide101in this exemplary embodiment can suppress loss caused by a step which is formed in a manufacturing process.

The third exemplary embodiment of the present invention will be described. There is shown inFIG. 8a bird's eye view of an exemplary configuration of an optical waveguide102in the third exemplary embodiment of the present invention.

The optical waveguide102in the third exemplary embodiment is different from the optical waveguide100in the first exemplary embodiment in a point that it further possesses a first joint60extended with approximately the same width as the first end11and a second joint70extended with approximately the same width as the sixth end43and the fourth end23. As shown inFIG. 9, the manufacturing method of the optical waveguide102is similar to the manufacturing method of the optical waveguide100.

The first joint60has one end which is connected with the first end11and the other end with which the optical waveguide of another optical functional device or the like can be connected with. Similarly, the second joint70has one end which is connected with the fourth end23and the sixth end43and the other end with which another optical device or the like can be connected with. Henceforth, the other end by which the first joint60can be connected with the optical waveguide of another optical functional device or the like is indicated as a seventh end61for illustrative purpose. Similarly, the other end by which the second joint70can be connected with another optical device or the like is indicated as an eighth end71.

A calculation result about the optical waveguide102in this exemplary embodiment is indicated below. InFIG. 10, a calculation result of dependence of transmission loss to the width of the first rib10and the second rib20(rib width) in the schematic illustration ofFIG. 10before and behind the barrier layer30(when its thickness is 3 μm) is shown. Here, calculation has been made supposing that a shape of the principal mode of light at the boundary surface between the second end12and the barrier layer30is an ellipsoid shape of 1 μm in height and 4 μm in width. As shown inFIG. 10, when the width of the third end22is expanded into 4 μm relative to the width of the principal mode of light of 4 μm, transmission loss can be reduced to a level of 0.05 dB or less. Further, a difference in transmission loss between TE (TE wave: Transverse Electric Wave) and TM (TM wave: Transverse Magnetic Wave) inFIG. 10can be reduced. That is, it is found that polarization dependence of transmission loss of guided light is reduced by making a rib width wider than 1.2 μm described in non-patent document 2 to 4 μm. Moreover, coupling loss to higher-order modes becomes small because a rib width is made wider as mentioned above, and thus the taper structures901-1and901-2before and behind a barrier layer903as shown inFIG. 18just like non-patent document 2 is not needed to be provided.

Dependence of transmission loss to the thickness of the barrier layer30when rib widths of the first rib10and the second rib20are 4 μm is shown inFIG. 11. As shown inFIG. 11, by making thickness of the barrier layer30thin as far as possible, transmission loss of guided light can be reduced, and, in addition, polarization dependence can also be reduced.

Dependence of transmission loss of guided light to the width of the third end22(rib width) and the width of the fifth end42(tip width) is shown inFIG. 12. As shown in the comparative example in the left graph inFIG. 12, when a rib width is small (1.3 μm), transmission loss of 0.04 dB is caused if the tip width is 0.4 μm, for example. On the other hand, when a rib width is wide and is 4 μm as is the case with this example shown in the right graph ofFIG. 12, transmission loss can be reduced to 0.02 dB which is about half of the above-mentioned transmission loss when the tip width is 1.3 μm.

Because the fifth end42has a limited width, the shape of a light mode changes greatly between before and behind its tip position generally, and thus transmission loss is caused. However, the wider the width of the third end22is, the more light is shut in the first layer21, and electromagnetic field distribution of light is also shut inside the first layer21mostly. For this reason, in the structure of this example in the right inFIG. 12, transmission loss can be suppressed compared with the comparative example in the left inFIG. 12, because electromagnetic field distribution of light rarely changes between before and behind the tip position of the fifth end42.

In the example 1, the widths of the fifth end42and the third end22are 0.4 μm and 4 μm, respectively. In order to obtain the above-mentioned effect, it is desirable that the width of the third end22is 10-20 times of the width of the fifth end42.

InFIG. 13, there is shown dependence of transmission loss of guided light to lateral displacement of the fifth end42from the center of the third end22. There is a possibility that the fifth end42shifts in a lateral direction (width direction) by deviation (about 50 nm) of mask positions due to precision issues at the time of production. However, because of the structure in which a rib width of the first layer21is wide sufficiently, a mask lateral displacement dependence of transmission loss can be suppressed as shown inFIG. 13. When the biggest shift of 50 nm occurs, while loss increases up to 0.06 dB in the comparative example in the left inFIG. 13(an example in which the rib width of the third end22is 1.3 μm), loss can be made small in the structure of this example in the right inFIG. 13(an example in which the rib width of the third end22is 4.0 μm) to one third of that of the comparative example.

As one usage example of the optical waveguide102in this exemplary embodiment, a case where an optical functional device is connected to the seventh end61and an optical fiber is connected to the eighth end71will be described below. InFIG. 14, there is shown an optical waveguide103in this example. Hereinafter, description will be made supposing that the shape of the principal mode of light in the side of the seventh end61is of a circular shape having a diameter of 1 μm and the principal mode of light in the side of the eighth end71is of a circular shape of 4 μm diameter.

As shown inFIG. 14, the cross section of the first joint60at the seventh end61has approximately the same shape as the optical waveguide of an optical functional device which is expected to be connected, and is of 1 μm square in this example. Further, the thickness of the slab layer1is made to be approximately the same thickness as the slab layer of an optical functional device to be connected. By making it such structure, the optical waveguide of an optical functional device which is expected to be connected with the optical waveguide102in this exemplary embodiment can be produced simultaneously by etching, and thus manufacturing can be simplified.

In addition, the eighth end71of the second joint70is of 4 μm square that corresponds to the principal mode of light guided in an optical fiber which is expected to be connected (Single Mode Fiber (SMF) is indicated in the figure as an example). Because the mode of light in the side of the optical fiber is centrosymmetric, the mode of light in the side of the optical waveguide is also made to be centrosymmetric in order to reduce coupling loss. Because a mode diameter in the side of an optical fiber is generally larger than that of the optical waveguide side, the eighth end71can be made larger up to about 7 μm square. Further, coupling loss can be also reduced by fitting only the height of the eighth end71or the width of the second layer41to the mode diameter of the optical fiber side. However, when making only the height or the width fit thus, coupling loss with an optical fiber increases due to deterioration of the inner symmetry if the ratios between height and width differ greatly.

Here, the structure when applying the technology described in non-patent document 2 (taper structures901-1and901-2inFIG. 18) and the structure of example 2 will be compared. Here, a code used in this example is used appropriately for a common structure. When making the principal mode of light in the first end11side be a circular shape of 1 μm diameter, the principal mode of light in the side of the fourth end23and the sixth end43be a circular shape of 4 μm diameter and the thickness of the barrier layer30be 3 μm, taper structures of 500 μm are needed in before and behind the barrier layer30, respectively, when the technology described in non-patent document 2 is applied Further, for conversion of the principal mode of light in the height direction, it is needed to have an optical waveguide of 800 μm long (it corresponds to the second rib20) being connected with one of the above-mentioned taper structures and including the first layer21having a gradually-widened width and a multilayer structure to make the diameter of the principal mode expand in the height direction (this corresponds to the second layer41) laminated on the first layer21. That is, about 1800 μm of length is needed for the structure related to the conversion of the diameter of the principal mode of light.

However, in the structure of the optical waveguide102in this example, one taper structure existing between the barrier layer30and the second rib20among the above-mentioned taper structures is unnecessary, and 500 μm of shortening can be made, first. Furthermore, when the first layer21is made to have the third end22of 4 μm and have a structure in which its width increases gradually up to the width of the second joint70that is connected with an optical fiber from the third end22to the fourth end23, the length of the second rib20can be suppressed to be 500 μm, and thus 300 μm of shortening can be made. In other words, 800 μm of shortening can be made totally. In this structure, loss has been improved by 0.15 dB as a whole device compared with the structure when applying the technology described in non-patent document 2 mentioned above.

Also in the optical waveguide100of the first exemplary embodiment and the optical waveguide101in the second exemplary embodiment, the cross section of the first end11can be made to have approximately the same shape as that of the optical waveguide of an optical functional device which is expected to be connected, as shown inFIG. 15. By making it such structure, manufacturing can be simplified by a reason that has been mentioned earlier in the example 2.

Making it such structure may be applied not only to an optical functional device, but also to other optical members and optical devices which are expected to be connected, or to the second joint70.

Moreover, various modifications to these exemplary embodiments will be readily apparent to those skilled in the art, and the generic principles and specific examples defined herein may be applied to other embodiments without the use of inventive faculty. Therefore, the present invention is not intended to be limited to the exemplary embodiments described herein but is to be accorded the widest scope as defined by the limitations of the claims and equivalents. Further, it is noted that the inventor's intent is to retain all equivalents of the claimed invention even if the claims are amended during prosecution.