Conventionally, an optical waveguide structure in which an optical waveguide configured to be coupled to an optical fiber is layered on a substrate formed with a groove for positioning the optical fiber, and an optical-waveguide-type optical module including the optical waveguide structure have been known (see the Patent Publication 1 below).
Further, conventionally, an optical waveguide structure which is a component of an optical-waveguide-type optical module, an optical-waveguide-type optical module, and an optical fiber array configured to be coupled to an optical waveguide so as to form an optical module have been known. Particularly, the optical waveguide structure, the optical-waveguide-type optical module, and the optical fiber array, each of which having a groove for supporting an optical fiber, have been known (see the Patent Publication 1 below).
Referring to FIGS. 17-19, a first example of a conventional optical waveguide structure will be explained. FIG. 17 is a top plan view showing an optical-waveguide-type optical module including a conventional optical waveguide structure. FIG. 18 is a fragmentary enlarged cross-sectional view taken along the line XVIII-XVIII in FIG. 17, and FIG. 19 is a fragmentary enlarged cross-sectional view taken along the line XIX-XIX in FIG. 17.
As shown in FIGS. 17-19, an optical-waveguide-type optical module 200 having a single upstream optical fiber 202 extending longitudinally, eight downstream optical fibers 204 spaced longitudinally from the upstream optical fiber 202 and arranged laterally relative to each other, and an optical waveguide structure 206 for transmitting light through the upstream optical fiber 202 to the downstream optical fibers 204. The upstream optical fiber 202 and the downstream optical fibers 204 include respective cores 202a, 204a extending longitudinally.
The optical waveguide structure 206 has a substrate 212 on which a single upstream groove 208 extending longitudinally and eight downstream grooves 210a-210h extending longitudinally and spaced longitudinally from the upstream groove 208 are provided, and an optical waveguide 214 layered on the substrate 212 between the upstream groove 208 and the downstream grooves 210a-210h. The upstream optical fiber 202 is positioned on the upstream groove 208, and the downstream optical fibers 204 are positioned on the downstream grooves 210a-210h. 
The optical waveguide 214 includes a lower cladding 214a layered on the substrate 212, a core 214b formed on the lower cladding 214a, and an upper cladding 214c layered on the lower cladding 214a and the core 214b. The core 214b of the optical waveguide 214 is formed so that, when the optical fibers 202, 204 are supported and positioned on the upstream groove 208 and the downstream grooves 210a-210h, the core 214b of the optical waveguide 214 is aligned with the cores 202a, 204a of the optical fibers 202, 204 at the same level in an vertical direction.
Further, for light transmission between the upstream optical fiber 202 positioned on the upstream groove 208 and each of the downstream optical fibers 204 positioned on the downstream grooves 210a-210h, the core 214b of the optical waveguide 214 has a single upstream port 220 aligned with the upstream groove 208 and eight downstream ports 222 each aligned with the respective downstream grooves 210a-210h. In the illustrated optical waveguide structure 206, the core 214b of the optical waveguide 214 extends from the single upstream port 220, is branched toward a downstream side, and terminates at the eight downstream ports 222. The optical waveguide 214 has an upstream portion 224a adjacent to the upstream port 220, an intermediate portion 224b between the upstream port 220 and the downstream ports 222, and a downstream portion 224c adjacent to one of the downstream ports 222.
Light transmitted through the single upstream optical fiber 202 is transmitted from the upstream port 220 to the optical waveguide 214 and branched toward the downstream side. Then, the branched lights are transmitted from the eight downstream ports 222 to the eight downstream optical fibers 204. In this case, the optical-waveguide-type optical module 200 serves as an optical splitter. On the contrary, when light is traveled in the opposite direction from the downstream optical fibers 204 to the upstream optical fiber 202, the optical-waveguide-type optical module 200 serves as an optical coupler.
Referring to FIGS. 20-22, a second example of the conventional optical waveguide structure which is a component of an optical-waveguide-type optical module will be explained. FIG. 20 is a top plan view showing an optical-waveguide-type optical module including the conventional optical waveguide structure. FIG. 21 is a fragmentary enlarged cross-sectional view taken along the line XXI-XXI in FIG. 20, and FIG. 22 is a fragmentary enlarged cross-sectional view taken along the line XXII-XXII in FIG. 20.
As shown in FIGS. 20-22, an optical-waveguide-type optical module 300 has a single upstream optical fiber 302 extending longitudinally, eight downstream optical fibers 304 spaced longitudinally from the upstream optical fibers 302 and arranged laterally relative to each other, and an optical waveguide structure 306 for supporting the upstream optical fiber 302 and the downstream optical fibers 304 and transmitting light through the single upstream optical fiber 302 to the eight downstream optical fibers. The optical-waveguide-type optical module 300 further has two fiber-holding lids 308a, 308b which respectively hold the upstream optical fiber 302 and the downstream optical fibers 304 against the optical waveguide structure 306, and an adhesive 310 filled between any two of the optical fibers 302, 304, the optical waveguide structure 306 and the fiber-holding lids 308a, 308b to fix them relative to each other.
The upstream optical fiber 302 and the downstream optical fibers 304 have respective cores 302a, 304a extending longitudinally. The optical waveguide structure 306 has a substrate 312 and an optical waveguide 314 layered thereon. The substrate 312 has an upper surface 316 with a lateral width W, and a plurality of grooves 318 for supporting the upstream optical fiber 302 and the downstream optical fibers 304 are formed in the upper surface 316. The optical waveguide 314 includes a core 314a which is formed so that, when the optical fibers 302, 304 are supported and positioned on the grooves 318, the core 314 of the optical waveguide 314 is aligned with the cores 302a, 304a of the optical fibers 302, 304. Each of the fiber-holding lids 302, 304 has the same width as that of the substrate 312, and is provided with (a) contact groove(s) 322 contacting with the optical fiber(s) 302, 304.
Light transmitted through the single upstream optical fiber 302 is transmitted to the optical waveguide 314, and branched toward a downstream side. Then, the branched lights are transmitted to the eight downstream optical fibers 304. In this case, the optical-waveguide-type optical module 300 serves as an optical splitter. On the contrary, when light is traveled in the opposite direction from the downstream optical fibers 304 to the upstream optical fiber 302, the optical-waveguide-type optical module 300 serves as an optical coupler.
When light is transmitted from the upstream optical fiber 302 to the optical waveguide 314 and transmitted from the optical waveguide 314 to the downstream optical fibers 304, a loss of optical power to be transmitted, called an insertion loss, is caused.
Patent Publication 1: Japanese Patent Laid-Open Publication No. 11-125731
In the above optical-waveguide-type optical module 200 which is the first conventional example, when light is transmitted from the upstream optical fiber 202 to the optical waveguide 214 and transmitted from the optical waveguide 214 to the downstream optical fibers 204, a loss of optical power to be transmitted, called an insertion loss, is caused. The insertion loss is a ratio of a downstream output optical power (Po) relative to an upstream input optical power (Pi) expressed in deci Bell unit, i.e., (10 log10 (Po/Pi)). An amount of the insertion loss of the optical-waveguide-type optical module 200 is preferable as small as possible. In a case of the optical module according to the present invention where there is no gain such as amplifying performance, i.e., Po<Pi, a value of the insertion loss expressed in deci Bell unit based on the above formula is negative. When there is no insertion loss, the value thereof based on the above formula is 0 (zero). Thus, in this specification, a small insertion loss means that a negative value calculated by the above formula is large, namely, an absolute value thereof is close to 0 (zero), as with common practice. Such insertion loss can be applied to a case including both of an insertion loss and a gain. In this case, the insertion loss can be considered after a part of the gain is separated. Therefore, an optical waveguide structure is needed, which structure can reduce an amount of insertion loss of an optical-waveguide-type optical module thereof as compared with the conventional optical waveguide structure.
An amount of insertion loss of the optical-waveguide-type optical module 200 shown in FIGS. 17-19, i.e., that of insertion loss between the upstream optical fiber 202 and the optical waveguide 214 or that of insertion loss between the optical waveguide 214 and the downstream optical fibers 204, may be changed depending on changes in an environmental temperature. Particular, since an optical splitter and an optical coupler which are examples of the optical-waveguide-type optical module are used as a part of the Internet with an optical fiber network disposed outside, an environmental temperature thereof may vary, for example, from −40° C. to +85° C. When insertion loss of such an optical-waveguide-type optical module is fluctuated due to variations in the environmental temperature, the optical module may not be able to give its original performance. The fluctuation of the insertion loss of the optical-waveguide-type optical module due to the change in the environmental temperature is preferably as small as possible. Thus, it is desirable to provide an optical waveguide structure capable of reducing fluctuation of an insertion loss of an optical-waveguide-type optical module as compared with that of the conventional optical-waveguide-type optical module.
In the above optical-waveguide-type optical module 300 which is the second conventional example, the upstream fiber-holding lid 308a is likely to be fixed to the optical fiber in a state laterally inclined relative to the substrate 312 (see FIG. 22). In this case, on the opposite sides of the upstream optical fiber 302, distances between the fiber-holding lid 308a and the upper surface 316 of the substrate 312 are different from each other as well as thicknesses of the adhesive 310 are different from each other.
As mentioned above, an optical splitter and an optical coupler, which are examples of the optical-waveguide-type optical module 300, are used as a part of the Internet with the optical fiber network placed outdoor. An environmental temperature around the optical module 300 may be changed, for example, from −40° C. to +85° C. When the environmental temperature is changed, each of the substrate 312, the fiber-holding lid 308a and the adhesive 310 expands and contracts according to their thermal expansion coefficients which are different from each other. Thus, when thicknesses of the adhesive 310 are different from each other on the opposite sides of the upstream optical fiber 302 and the environmental temperature is changed, the upstream optical fiber is subjected to uneven stress, resulting in deterioration of an insertion loss. Fluctuation of the insertion loss of the optical-waveguide-type optical module 300 when the environmental temperature is changed is preferably as small as possible.
Thus, there is a need to provide an optical waveguide structure which is a component of an optical-waveguide-type optical module and is capable of reducing fluctuation of an insertion loss thereof when an environmental temperature is changed. Further, there is a need to provide an optical-waveguide-type optical module capable of reducing fluctuation of an insertion loss thereof when an environmental temperature is changed.
Furthermore, there is a need to provide an optical fiber array configured to be coupled to an optical waveguide so as to form an optical module, the optical fiber array being capable of reducing fluctuation of an insertion loss of the optical module when an environmental temperature is changed.
It is therefore a first object of the present invention to provide an optical waveguide structure capable of reducing an insertion loss of an optical-waveguide-type optical module including the optical waveguide structure as compared with the conventional optical waveguide structure, and to provide the optical-waveguide-type optical module itself.
Further, it is a second object of the present invention to provide an optical waveguide structure capable of reducing fluctuation of an insertion loss of an optical-waveguide-type optical module including the optical waveguide structure when an environmental temperature is changed, and to provide the optical-waveguide-type optical module itself.
Further, it is a third object of the present invention to provide an optical fiber array configured to be coupled to an optical waveguide so as to form an optical module, the optical fiber array being capable of reducing fluctuation of an insertion loss of the optical module when an environmental temperature is changed.