Abstract:
An optical fiber apparatus includes an optical fiber, and a demultiplexing/multiplexing unit for demultiplexing or multiplexing at least a light wave of at least a wavelength with a sufficiently narrow wavelength spectrum that is determined by a resolving power thereof, such as a Fabry-Perot etalon. The demultiplexing/multiplexing unit is provided or formed directly on at least an end face of the optical fiber. An optical detector array including a plurality of optical detectors for detecting demultiplexed light waves may be provided on a light emergence surface of of the demultiplexing/multiplexing unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical fiber apparatus provided with a demultiplexing/multiplexing unit on its fiber&#39;s end surface, such as an optical fiber provided with an optical detector array having a demultiplexing function for separating wavelength multiplexed signals of a wavelength division multiplexing (WDM) optical transmission and for receiving separated optical signals by respective optical detectors, an optical detecting apparatus provided with a demultiplexing-multiplexing unit on its light receiving surface, and an optical transmission system using the same. 
     2. Related Background Art 
     Conventionally, the following combination structures were proposed as an optical detector having a demultiplexing function for demultiplexing wavelength multiplexed signals and for receiving respective demultiplexed optical signals, as disclosed in Japanese Laid-Open Patent Nos. 8(1996)-82711 and 8(1996)-211237. In one structure, a device having a combination of a branching function and an optical filter (band-pass) function is used. In another structure, so-called array-waveguide diffraction gratings are used as a demultiplexer and an optical detector (or an optical detector array) is combined with the diffraction gratings. In the array-waveguide diffraction gratings, a plurality waveguides having different optical lengths are combined, and different wavelengths are coupled to different output waveguides due to an interference effect. 
     The prior art device of Japanese Laid-Open Patent No. 8(1996)-82711, however, has the following technical disadvantages since optical separation is effected using multiple reflections: 
     1. External light must be collimated and then input; 
     2. The angle of input light must be set to a given angle; and 
     3. Performance of the optical filter must be varied according to its location. 
     Further, the prior art device of Japanese Laid-Open Patent No. 8(1996)-211237 has the following technical disadvantages since demultiplexing is effected using interference between different optical lengths: 
     1. An optical fiber must be optically coupled to the waveguide; 
     2. A possibility exists that variation in a waveguide width of the array-waveguide gratings adversely affects characteristics; 
     3. A thin waveguide device must be fixed to a fiber end; and 
     4. A slab waveguide portion must be precisely processed to uniformly distribute light. 
     In addition to those disadvantages, in both prior art devices, an optical coupling system is needed between the optical fiber and the demultiplexer, and a portion having a separating function (spatial separation) and a size larger than the optical fiber is needed. 
     Furthermore, there is not yet provided an optical fiber (typically, a plastic optical fiber having a relatively large core diameter) which has a demultiplexing/multiplexing unit provided directly on its end portion to solve the above disadvantages. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an optical fiber apparatus including a demultiplexing/multiplexing unit on an end face of the optical fiber, such as an optical fiber provided with an optical detector having a demultiplexing function for separating wavelength multiplexed signals of a wavelength division multiplexing (WDM) optical transmission and for receiving respective separated optical signals, an optical detecting apparatus provided with a demultiplexing/multiplexing unit on its light receiving surface, and an optical transmission system using the same. 
     An optical fiber apparatus for achieving the object of the present invention includes an optical fiber and a demultiplexing/multiplexing unit for demultiplexing or multiplexing a light wave of at least a wavelength with a sufficiently narrow wavelength spectrum that is determined by a resolving power of the demultiplexing/multiplexing unit. The demultiplexing/multiplexing unit is provided or formed directly on at least an end face of the optical fiber. 
     Typically, the demultiplexing/multiplexing unit is provided on the end face of the optical fiber, so that needed demultiplexing and receiving are performed within an area of a size of a core of the optical fiber. For example, transmission wavelengths of the demultiplexing/multiplexing unit vary depending on its light emergence places, and optical detectors are respectively arranged at those places. Thus, where wavelength multiplexed optical signals transmitted through the optical fiber are received by a receiver per wavelength, precision needed for optical couplings and the number of positional alignment processes can be reduced. 
     In the structure of the present invention, a plurality of optical filters with different transmission wavelengths need not be finely provided in a small area, in contrast with the conventional apparatus. For example, when an etalon with predetermined opposite end surfaces is used, a demultiplexing/multiplexing unit whose transmission wavelengths vary depending on its light emergence places can be readily fabricated. Such a structure can be effectively built especially where the optical fiber has a relatively large core diameter or size. 
     Based on the above fundamental structure, the following specific structures are possible with the following technical advantages. 
     Typically, the demultiplexing/multiplexing unit demultiplexes or multiplexes a plurality of light waves at different wavelengths with sufficiently narrow wavelength spectra that are determined by the resolving power of the demultiplexing/multiplexing unit. 
     The demultiplexing/multiplexing unit is a Fabry-Perot etalon whose optical length varies along a direction approximately perpendicular to an optical-axial direction of the optical fiber. For example, the unit may be a wedge-shaped Fabry-Perot etalon including reflective mirrors wherein spacings between the reflective mirrors gradually vary along the direction approximately perpendicular to the optical-axial direction of the optical fiber. In these etalons, transmission wavelengths vary depending on places along the direction approximately perpendicular to the optical-axial direction of the optical fiber. 
     The demultiplexing/multiplexing unit may include a unit for preventing multiple reflection bridging paths of demultiplexed light waves such that crosstalk between adjacent demultiplexed light waves can be prevented. In such a structure, channels in the etalon can be separated from each other, and unwanted leaks of light between channels can be lowered. Thus, demultiplexing performance can be effectively improved. 
     The wedge-shaped Fabry-Perot etalon may be composed of an optical material having non-parallel opposite end surfaces, and a uniform refractive index and reflective films provided on the non-parallel opposite end surfaces. 
     The optical fiber apparatus may further include an optical detector array including a plurality of optical detectors for detecting demultiplexed light waves, and the demultiplexing/multiplexing unit may be a wedge-shaped Fabry-Perot etalon including a reflective film provided on the end face of the optical fiber and a reflective film provided on an end surface of the optical detector array. The reflective films are set in a predetermined non-parallel relationship with a spacing between the reflective films. In such a structure, no special material for the etalon is needed, so that the structure can be simplified. 
     The wedge-shaped Fabry-Perot etalon may further include a jig for placing the optical fiber and the optical detector array along a common axis, or a spacer for placing the optical fiber and the optical detector array with a spacing between the optical fiber and the optical detector array, to achieve the predetermined non-parallel relationship. In this case, a control unit for varying the predetermined non-parallel relationship, such as a piezoelectric element, may be further provided. 
     The demultiplexing/multiplexing unit may include parallel reflective films and an optical material which are sandwiched between the reflective films and have a predetermined refractive-index distribution. In such a structure, even when the reflective films are parallel, the same effect as that of the above etalon can be achieved since the optical material has the refractive-index distribution. The predetermined refractive-index distribution may be a monotonously-varying linear distribution, a stepwise distribution, a monotonously-varying stepwise distribution, or a randomly-varying stepwise distribution. The optical material having the predetermined refractive-index distribution may be a graded-index (GI) optical fiber. 
     The demultiplexing/multiplexing unit may include two optical materials having mutually-complementary surfaces, flat end surfaces and different refractive indices, respectively, and reflective films provided on the opposite flat end surfaces of the optical materials, respectively. The mutually-complementary surfaces are brought into a tight contact with each other to form an interface. The mutually-complementary surfaces may be stepwise mutually-complementary surfaces. 
     The optical fiber apparatus may further include an optical detector array including a plurality of optical detectors for detecting demultiplexed light waves, and the demultiplexing/multiplexing unit may comprise a Fabry-Perot etalon including a reflective film provided on the end face of the optical fiber and a reflective film provided on an end surface of the optical detector array. 
     The demultiplexing/multiplexing unit includes a diffraction grating formed on an end face of the optical fiber slantingly relative to an optical axis of the optical fiber. The diffraction grating diffracts light in different directions per wavelength. 
     The optical fiber apparatus may further include an optical detector array including a plurality of optical detectors for detecting demultiplexed light waves, and the optical detector array may be provided on a side surface at an end portion of the optical fiber to which diffracted light is guided from the diffraction grating. In this case, when a lens for converging light is provided between the diffraction grating and the optical detector array, wavelength separation due to dispersion of the grating can be achieved in a short range. 
     The demultiplexing/multiplexing unit may include a reflective surface, formed on an end face of the optical fiber for reflecting and deflecting light transmitted through the optical fiber by 45° slantingly relative to an optical axis of the optical fiber, and an optical filter array including a plurality of optical filters for selecting light at different wavelengths. The optical filter array may be provided on a side surface of the optical fiber to which reflected light is guided from the reflective surface. In this case, an optical detector array including a plurality of optical detectors for detecting light waves demultiplexed by the optical filter array may be provided on the optical filter array. 
     The demultiplexing/multiplexing unit may include a core-extending portion, having a refractive index different from a refractive index of a core of the optical fiber and provided on an end face of the optical fiber, and a diffraction grating formed at an interface between the core-extending portion and the core of the optical fiber. 
     The core-extending portion may be a core-expanding portion which gradually expands from the end face of the optical fiber. In this structure, diffracted light at different wavelengths can be easily guided to different places on an expanded end face of the core-expanding portion. 
     The core-extending portion may include a plurality of diffraction gratings arranged such that wavelength multiplexed light transmitted through the optical fiber is successively diffracted by each of the diffraction gratings. 
     An optical detector array including a plurality of optical detectors for detecting light waves demultiplexed by the demultiplexing/multiplexing unit may be provided on the demultiplexing/multiplexing unit, and the demultiplexed light waves diffracted by the diffraction grating may be guided to the optical detectors. 
     The demultiplexing/multiplexing unit may include a deflecting optical switch for deflecting light waves in different directions depending on the light wave&#39;s wavelength or incident timing. The deflecting optical switch may include a control unit for varying a deflection function of the deflecting optical switch. The deflecting optical switch may perform a deflection function using an acousto-optical effect, for example. 
     The optical fiber apparatus may further comprise an optical detector array including a plurality of optical detectors for detecting light waves demultiplexed by the demultiplexing/multiplexing unit, which is provided on the demultiplexing/multiplexing unit or an end portion of the optical fiber. 
     A shape of an end face of the optical fiber may be approximately coincident with a shape of a light-receiving area of the optical detector array. In this structure, the demultiplexing/multiplexing unit can be made compact in size, and the size of a signal receiving system (such as optical fiber, demultiplexing/multiplexing unit, and optical detectors) can also be reduced. 
     The optical fiber may include a core-expanding portion formed on an end face of the optical fiber. A core size is gradually expanded from the end face of the optical fiber. The core-expanding portion may include a lens for converging light formed in the core-expanding portion. In this structure, the multiplexing number and a detection area of each optical detector in the detector array can be increased. 
     A shape of the optical detector array having a light-receiving area may be approximately coincident with a shape of an expanded end face of the core-expanding portion. 
     Further, the optical fiber may include a core whose shape is a square or rectangular at least at an end portion of the optical fiber. In this structure, the square core can be effectively coupled to the optical detector array, and the field distribution of emerging light from the optical fiber can be readily coincident with the incident field of the demultiplexing/multiplexing unit. 
     The optical fiber may be a plastic optical fiber which is formed of optical material, such as polymer and synthetic resin, which is optically transparent. 
     The demultiplexing/multiplexing unit may be provided or formed only on the end face of said optical fiber or on the end face and a side surface of the optical fiber. 
     A core of the optical fiber may be removed at an end portion of the optical fiber with a clad being partially left, and the demultiplexing/multiplexing unit may be provided on the partially left clad. 
     A core of the optical fiber may also be removed at an end portion of the optical fiber with a clad being entirely left, and the demultiplexing/multiplexing unit may be provided in the entirely left clad. 
     At least the demultiplexing/multiplexing unit may be integrally molded with an end portion of the optical fiber. In this case, the demultiplexing/multiplexing unit and the optical detector array may be integrally molded with an end portion of the optical fiber. Such an optical fiber with the integrated optical detector array and demultiplexing/multiplexing unit can be readily mounted to a board on a receiver side. 
     An optical detecting apparatus for achieving the object of the present invention includes an optical detecting unit including a plurality of optical detectors and a demultiplexing/multiplexing unit for demultiplexing or multiplexing a plurality of light waves at different wavelengths with sufficiently narrow wavelength spectra that are determined by the resolving power of the demultiplexing/multiplexing unit. The demultiplexing/multiplexing unit is provided or formed directly on at least a light-receiving surface of the optical detecting unit. Such an optical detecting apparatus can be made compact in size. 
     An optical transmission system for achieving the object of the present invention includes (1) an optical fiber through which wavelength multiplexed optical signals are transmitted where a wavelength interval of the wavelength multiplexed optical signals is Δλ 1  and a wavelength width occupied by each optical signal is Δλ 2  which is not larger than Δλ 1 , and (2) a demultiplexing/multiplexing unit for demultiplexing or multiplexing the wavelength multiplexed optical signals with sufficiently narrow wavelength spectra that are determined by the resolving power of the demultiplexing/multiplexing unit. The narrowness is preferably about equal to Δλ 2 . The demultiplexing/multiplexing unit is provided or formed directly on at least an end face of the optical fiber. 
     An optical transmission system for achieving the object of the present invention includes a light transmission line, an optical detecting unit including a plurality of optical detectors, and a demultiplexing/multiplexing unit. Wavelength multiplexed optical signals are transmitted through the transmission line and received by the optical detecting unit where a wavelength interval of the wavelength multiplexed optical signals is Δλ 1  and a wavelength width occupied by each optical signal is Δλ 2  which is not larger than Δλ 1 . The demultiplexing/multiplexing unit demultiplexes or multiplexes the wavelength multiplexed optical signals with sufficiently narrow wavelength spectra that are determined by a resolving power of the demultiplexing/multiplexing unit. The narrowness is about equal to Δλ 2 . The demultiplexing/multiplexing unit is provided or formed directly on at least a light receiving surface of the optical detecting unit. 
     The demultiplexing/multiplexing unit may include a light entrance surface for receiving light from the light transmission line and a light emergence surface, and the demultiplexing/multiplexing unit may be set such that demultiplexed optical signals of different wavelengths at least partially emerge at different positions of the light emergence surface, respectively. 
     The optical detectors may be provided at different positions of the light emergence surface to receive the optical signals at different wavelengths, respectively. The optical detectors may be provided on the light emergence surface such that at least one optical signal of the wavelength mutliplexed optical signals can be received separately. The optical detectors may be provided on the light emergence surface such that all the wavelength mutliplexed optical signals can be received separately. 
     These advantages and others will be more readily understood in connection with the following detailed description of the preferred embodiments in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of a first embodiment of the present invention. 
     FIG. 1B is a perspective view illustrating a wedge-shaped Fabry-Perot etalon used in the first embodiment. 
     FIG. 1C is a front view illustrating an end portion of the optical fiber in the first embodiment. 
     FIG. 2 is a cross-sectional view illustrating the structure of a modification of the first embodiment in which plural optical fibers are connected to the etalon in place of an optical detector array. 
     FIG. 3 is a view illustrating characteristics of the wedge-shaped etalon used in the first embodiment, which are determined by a resolving power of the etalon. 
     FIG. 4 is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of a second embodiment of the present invention. 
     FIG. 5A is a view along an optical-axial direction of an optical fiber illustrating the structure of a third embodiment of the present invention. 
     FIG. 5B is a cross-sectional view taken along line A-A′ in FIG. 5A illustrating a support jig. 
     FIG. 6 is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of a fourth embodiment of the present invention. 
     FIG. 7A is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of a fifth embodiment of the present invention. 
     FIG. 7B is a view illustrating a refractive-index distribution of an optical material used in the fifth embodiment. 
     FIG. 7C is a view illustrating another refractive-index distribution of an optical material used in the fifth embodiment. 
     FIG. 8 is a view illustrating yet another refractive-index distribution of an optical material used in the fifth embodiment. 
     FIG. 9 is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of a sixth embodiment of the present invention. 
     FIG. 10A is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of a seventh embodiment of the present invention. 
     FIG. 10B is a view illustrating an optical material used in the seventh embodiment. 
     FIG. 11 is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of an eighth embodiment of the present invention. 
     FIG. 12 is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of a ninth embodiment of the present invention. 
     FIG. 13 is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of a tenth embodiment of the present invention. 
     FIG. 14 is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of an eleventh embodiment of the present invention. 
     FIG. 15 is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of a twelfth embodiment of the present invention. 
     FIG. 16 is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of a thirteenth embodiment of the present invention. 
     FIG. 17 is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of a fourteenth embodiment of the present invention. 
     FIG. 18 is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of a fifteenth embodiment of the present invention. 
     FIG. 19 is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of a sixteenth embodiment of the present invention. 
     FIG. 20 is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of a seventeenth embodiment of the present invention. 
     FIG. 21 is a cross-sectional view taken along an optical-axial direction of an optical fiber illustrating the structure of an eighteenth embodiment of the present invention. 
     FIG. 22A is a cross-sectional view of an optical fiber having a square core. 
     FIG. 22B is a view illustrating an optical detector array provided on an end face of the fiber of FIG.  22 A. 
     FIG. 23A is a cross-sectional view of an optical fiber having a rectangular core. 
     FIG. 23B is a view illustrating an optical detector array provided on an end face of the fiber of FIG.  23 A. 
     FIG. 24A is a cross-sectional view of an optical fiber having three separate rectangular cores. 
     FIG. 24B is a cross-sectional view of the optical fiber of FIG. 24A taken along its optical-axial direction. 
     FIG. 25 is a cross-sectional view of an optical fiber having three separate rectangular cores at its end portion, taken along its optical-axial direction. 
     FIG. 26 is a cross-sectional view of a coupler using optical fibers having a square core, taken along its optical-axial direction. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     A first embodiment of the present invention is shown in FIGS. 1A to  1 C. In FIG. 1A, illustrating a cross section of an optical fiber  1  taken along its optical axis, the optical fiber  1  consist of a core  1   a  having a circular cross section and a clad  1   b  therearound. Reflective film  2  and  3  are provided on non-parallel opposite end faces of an optical material  4 , respectively. An optical detector array  5  including a plurality of photodetectors  5   a - 5   n , such as pin photodiodes and avalanche photodiodes, is provided on the reflective film  3 . The optical material  4  and the reflective films  2  and  3  constitute a so-called Fabry-Perot etalon  9 . 
     The Fabry-Perot etalon  9  will be described in more detail. The optical material  4  of the etalon  9  has a profile of a wedge as illustrated in FIG.  1 B. Opposite end faces  4   a  and  4   b  of the optical material  4  are nearly parallel, but their non-parallel relationship is shown in an exaggerated manner in FIG.  1 B. The nonparallel faces  4   a  and  4   b  of the wedge shape illustrated in FIG. 1B form an angle θ therebetween which is determined from an interval d pd  between detectors  5   a - 5   n  of the photodetector array  5 , as described later. The reflective films  2  and  3  are formed on those faces  4   a  and  4   b . Reflectances of the reflective films  2  and  3  are determined from a wavelength resolving power required by a system to which this embodiment is applied. The size of the optical detector array  5  is approximately the same as that of a core diameter d core  of the optical fiber  1 , as illustrated in FIGS. 1A and 1C. FIG. 1C shows an end-face side of the optical fiber  1 . 
     The angle (θ) between the two faces  4   a  and  4   b  of the wedge-shaped etalon  9  and the interval (d pd ) between detectors  5   a - 5   n  of the optical detector array  5  will be described. The following relations must be met; 
      θ=tan −1 {[(N−1)Δλ/λ]/d e /d core } 
     
       
         d pd =[1/(N pd −1)]×d core /cos θ, 
       
     
     where N pd  is the number of optical detectors  5   a - 5   n , Δλ is the wavelength interval between wavelength multiplexed optical signals transmitted through the optical fiber  1 , N is the number of the multiplexed wavelengths, d core  is the core diameter of the optical fiber  1 , n is the refractive index of the optical material  4  of the etalon  9 , and d e  is the thickness of the etalon  9  (the thickness is not constant because of the wedge shape, but here the thickness is that of the thinnest portion), and λ is a center wavelength of the wavelength multiplexed signals. Further, FSR/(N−1)&gt;FSR/F must be met where FSR is the free spectral range of the etalon  9 , F is the finesse which is a function of the reflectance of the etalon  9  (reversely proportional to the reflectance). 
     The angle (θ) of the etalon  9 , reflectances of the reflective films  2  and  3 , interval (d pd ) between the optical detectors  5   a - 5   n  and so forth are set such that the above conditions are met. Thereby, each optical signal of the wavelength multiplexed signals transmitted through the optical fiber  1  can be received by each of the optical detectors  5   a - 5   n . FIG. 3 shows a transmission spectrum at each position on the etalon  9  corresponding to each detector. For example, where n=1.5, Δλ=1 nm, N=10 and d core =1 mm (such values can be taken, typically, in a plastic optical fiber for short-distance optical transmission), it can be known from the above relations that θ must be equal to 0.0257°. The angle θ is relatively small, but this value can be achieved when fabrication techniques can attain an optical flatness with a precision of about λ/10. 
     When all wavelengths of N-wavelength multiplexed light are not needed to be demultiplexed by the etalon  9 , i.e., M wavelengths (M&lt;N) of the N-wavlength multiplexed optical signals are to be selected, N and N pd  (the number of the optical detectors) in the above relations are respectively replaced by M to obtain the wedge angle (θ), interval between the detectors, reflectance of the etalon  9  and other values of this case. Only three wavelengths out of ten-wavelength multiplexed signals can be demultiplexed and received, for example. 
     Herein, glass having the refractive index of about 1.5 is used as the optical material  4 , but the above discussion is in principle independent of such material. Accordingly, so long as the angle between faces  4   a  and  4   b  can be regulated, the optical material  4  may be a semiconductor and material of the reflective films  2  and  3  may be a dielectric multi-layer or a semiconductor multi-layer, for example. 
     In FIG. 1A, the optical detector array  5  is provided on the end face of the etalon bonded to the end face of the optical fiber  1 . However, a structure as illustrated in FIG. 2 is possible. In FIG. 2, end faces of plural fibers  6  are bonded to the reflective film  3  on the etalon surface to connect the respective wavelengths to different paths. 
     In this embodiment the optical fiber  1  is the plastic optical fiber having its core diameter of 1 mm, but the size of the fiber is not limited thereto. Material of the optical fiber  1  also is not limited to a specific one. Both quartz and plastic fibers can be used. 
     Further, herein the etalon  9  and the detector array  5  are fixed to the end faces of the optical fiber  1  and the etalon  9  with appropriate adhesive, respectively, but it is preferable that no adhesive is used. 
     The operation of this embodiment will be described. When wavelength multiplexed optical signals travel through the optical fiber  1  and reach the etalon portion, respective signals of the wavelength multiplexed optical signals transmit through different portions of the etalon  9  since respective portions of the wedge-shaped etalon  9  are set to have different transmission wavelengths. Thus, the wavelength multiplexed optical signals are spatially separated in the etalon portion owing to a difference in the wavelength. Respective detectors  5   a - 5   n  of the detector array  5  placed at the back of the etalon  9  act to receive optical signals in different channels. 
     It is, however, not always necessary to receive all the signals of the wavelength multiplexed signals by different detectors, respectively. Various detecting manners are possible. In one case, only several signals of wavelength multiplexed signals are demultiplexed and received by different detectors. In another case, several signals are detected by a single detector. This can be achieved by enlarging a detecting region of such a detector. Those operations are common within an applicable scope to embodiments described later. 
     The present invention can also be applied to a light radiating structure as well as the above light detecting structure. Specifically, the above detector array is replaced by a light source array. In this case, different radiation spectra from respective light sources are multiplexed by the etalon portion to be coupled to the optical fiber. Such a structure is especially effective where a light source with a relatively wide radiation spectrum, such as LED, is used, since the respective spectral widths are sharpened through the etalon portion and multiplexed at the end face portion of the optical fiber. This is also common within an applicable scope to embodiments described later. 
     Second Embodiment 
     A second embodiment of the present invention is shown in FIG.  4 . In FIG. 4, showing a cross section of an optical fiber  1  taken along its optical axis, reflective films  12  and  13  are provided on opposite end faces of a wedge-shaped optical material  14 , respectively. A wedge-shaped etalon  19  consists of the wedge-shaped optical material  14  and the reflective films  12  and  13 . An optical detector array  15  including plural optical detectors  15   a - 15   n  is provided on the reflective film  13 . Reflection preventing structures  12   d  for preventing unwanted reflection are formed on an interface portion between the reflective film  12  and the optical material  14  at intervals of d pd . Positions of those structures  12   d  are shifted from positions opposingly facing the respective optical detectors  15   a - 15   n . A wedge angle (θ) of the etalon  19 , core diameter (d core ) of the optical fiber  1  and so forth have relations therebetween as described in the first embodiment. 
     In the second emodiment, unwanted reflection at the reflective film  12  can be prevented, and hence each light wavelength can be detected more accurately. FIG. 4 indicates undesired light, using a dotted line, which is likely to occur where no reflection preventing structure is provided. In the second embodiment, the reflection preventing structure  12   d  is formed by introducing a portion lacking the reflective film  12 . 
     The reflection preventing structure  12   d  may be provided on both or either of the reflective films  12  and  13 . The case of the formation on both sides is more effective than the case of the formation on either side, but the formation on either side can be preferable depending on the purpose of use. The reflection preventing structure may be provided not only on the reflective film but also in the optical material  14  for separating optical paths to the respective optical detectors  15   a - 15   n  from each other to construct a complete light blocking structure. Other structures and operations of this embodiment are the same as those of the first embodiment. 
     Third Embodiment 
     A third embodiment of the present invention is shown in FIGS. 5A and 5B. In FIG. 5A showing a cross section of a plastic optical fiber  1  taken along its optical axis, the optical fiber  1  includes a core having a core diameter of 1 mm. An optical detector array  25  includes plural optical detectors  25   a - 25   n  each having a light receiving surface slightly slanting relative to the end face of the optical fiber  1 . Reflective films  22  and  23  are provided on the end face of the optical fiber  1  and the slanting end surface of the detector array  25 , respectively. The optical fiber  1  and the optical detector array  25  are fixed in a predetermined relationship on a common axis by a support jig  26  with grooves  26   a  and  26   b . FIG. 5B shows the support jig  26  which has the groove  26   b  of a square cross section for fixing the optical fiber  1 , a groove for fixing the detector array  25  with a square cross section corresponding to the shape of the detector array  25 , and a V-shaped groove  26   a  for forming a space between the optical fiber  1  and the detector array  25 . 
     The detector array  25  has a cylindrical shape which is formed by cutting a cylinder along a direction inclining from a plane perpendicular to its central axis. The inclination angle is described later. 
     In the third embodiment, the reflective film  22  is formed on the end surface of the fiber  1 . Further, the reflective film  23  is provided on a surface of a wedge-shaped member on which the detector array  25  is provided. The optical fiber  1  with the reflective film  22  and the detector array  25  with the reflective film  23  are fixed to the support jig  26  such that a predetermined interval distance is formed between the two reflective films  22  and  23 . Thus, an etalon structure having the same function as the above-discussed wedge-shaped etalon is formed of those reflective films  22  and  23  and the space therebetween. 
     The inclination angle (θ) of the member on which the detectors  25   a - 25   n  are provided and so forth are determined by the following relations, similarly to those of the first embodiment: 
     
       
         θ=tan −1 {[(N−1)Δλ/λ]/d e /d core } 
       
     
     
       
         d pd =[1/(N pd −1)]×d core /cos θ, 
       
     
     where N pd  is the number of optical detectors  25   a - 25   n , Δλ is the wavelength interval between wavelength multiplexed optical signals transmitted through the optical fiber  1  and input into the demultiplexing element of this embodiment (i.e., the space between the two reflective films  22  and  23 ), N is the number of the multiplexed wavelengths, d core  is the core diameter of the optical fiber  1 , and d pd  is the interval between the optical detectors  25   a - 25   n . Here, n is equal to 1 (one) since the space between the reflective films  22  and  23  is air, and λ and d e  are the same as those in the first embodiment. 
     In the thus-formed structure, the transmission wavelengths of portions of the demultiplexing element (etalon) corresponding to the respective optical detectors  25   a - 25   n  vary depending on the space interval, similarly to the first embodiment. Accordingly, the respective detectors  25   a - 25   n  can detect signals at different wavelengths. 
     Where the support jig  26  is formed of an Si member, the support jig and a portion of the optical detector can be integrated. When long-wavelength light, for which an Si detector cannot be used, is to be treated, the support jig made of material coping therewith can be used with the same effect. 
     In this embodiment, the detector array  25  has a cylindrical profile which can be readily supported, but a cubic shape or the like can also be used so long as the support jig can be modified corresponding thereto. 
     Further, when the posture of the detector array can be controlled by using a piezoelectric element or the like (i.e., the inclination angle can be changed), a wavelength range to be demultiplexed can be varied and the wavelength guided to the optical detector can be changed. Other structures and operation of this embodiment are the same as those of the first embodiment. 
     Fourth Embodiment 
     A fourth embodiment of the present invention is shown in FIG.  6 . In FIG. 6, showing a cross section of a plastic optical fiber  1  taken along its optical axis, reflective films  32  and  33  are provided on the end face of the optical fiber  1  and the end surface of an optical detector array  35 , respectively. Spacers  36  and  37  are provided between the reflective films  32  and  33  such that a desired spacing can be set therebetween. In FIG. 6, the spacers  36  and  37  are composed of members having different lengths. The spacer, however, may be an annular member whose shape is appropriately formed corresponding to the cross section of the fiber  1  and the configuration of the detector array  35 . 
     The inclination angle (θ) of the surface of the dectetor array  35  relative to the optical axis and so forth are determined as determined in the third embodiment. The spacers  36  and  37  are fixed to the end face of the fiber  1  and the detector array  35  is fixed to the spacers  36  and  37 , so that the etalon having a desired spacing set by the inclination angle (θ) can be formed on the end face of the fiber  1 . In this structure, postures of mirrors (the reflective films  32  and  33 ) of the etalon are thus established by the spacers  36  and  37 . Therefore, the member for supporting the reflective film  33  (i.e., the detector array  35  in this case) need not have an inclined surface, though the detector array  35  illustrated in FIG. 6 has such an inclined surface. Thus, its fabcation can be facilitated. 
     Also in this embodiment, when the posture of the detector array  35  can be controlled by changing the length of a portion of the spacer by using a piezoelectric element or the like (i.e., the inclination angle can be changed), a wavelength range to be demultiplexed can be varied and the wavelength guided to the detector can be changed. Other structures and operation of this embodiment are the same as those of the first embodiment. 
     Fifth Embodiment 
     A fifth embodiment of the present invention is shown in FIGS. 7A and 7B. In FIG. 7A, showing a cross section of a plastic optical fiber  1  taken along its optical axis, reflective films  42  and  43  are provided on opposite end surfaces of an optical material  44 , respectively. The optical material  44  has a refractive-index distribution therein. 
     The optical material  44  is shaped into a cubic or cylindrical configuration. The refractive-index distribution is introduced into the optical material  44  by doping plastic material with dopant, for example. An etalon is constructed by providing the reflective films  42  and  43  on the opposite end surfaces of the optical material  44 . The refractive index in the optical material  44  has a distribution as illustrated in FIG.  7 B. 
     The refractive-index distribution meets the following relations to construct the etalon for guiding different transmission wavelengths to respective optical detectors  45   a - 45   n  of an optical detector array  45 . A change Δn(x) of the refractive-index distribuiton is given as follows: 
     
       
         θ=tan −1 {[(N−1)Δλ/λ]/d e /d core } 
       
     
     
       
         Δn(x)=n 0 (1+x/tan θ), 
       
     
     where N pd  is the number of optical detectors  45   a - 45   n , Δλ is the wavelength interval between wavelength multiplexed optical signals transmitted through the optical fiber  1  and input into the demultiplexing element of this embodiment, N is the number of the multiplexed wavelengths, d core  is the core diameter of the optical fiber  1 , and d pd  is the interval between optical detectors  45   a - 45   n . Here, x is an axis in a direction perpendicular to a traveling direction of light, n 0  is the lowest refractive index in the optical material  44 , and λ and d e  are the same as those in the first embodiment. 
     When the optical material  44  has such a distribution as illustrated in FIG. 7B, there can be provided an etalon which outputs different transmission wavelengths corresponding to places of the respective detectors  45   a - 45   n  of the detector array  45 . Thus, the wavelength multiplexed signals can be demultiplexed, and the demultiplexed signals are received the respective detectors  45   a - 45   n . The operation is the same as that of the above embodiments. 
     In the above structure, the refractive index of the optical material  44  is linearly changed along the x-axis, but the refractive index may be changed in a stepwise manner correspondingly to places of the detectors  45   a - 45   n , as illustrated in FIG.  7 C. In this case, the stepwise refractive-index distribution is set such that centers of respective refractive-index steps meet the above conditions. A dotted line in FIG. 7C indicates a line connecting the centers of steps. 
     Further, the refractive-index distribution is not necessarily monotonously changed. The distribution only needs to be determined according to a distribution of wavelengths received by the respective detectors  45   a - 45   n . FIG. 8 illustrates an example of a non-monotonous refractive-index distribution. Furthermore, an electro-optic material may also be used as the optical material  44 , and thus the refractive-index distribution may be changed by controlling an electric field applied to the electro-optic material. 
     Sixth Embodiment 
     A sixth embodiment of the present invention is shown in FIG.  9 . In FIG. 9 showing a cross section of a plastic optical fiber  1  taken along its optical axis, reflective films  52  and  53  are provided on opposite end surfaces of a graded-index (GI) plastic optical fiber  54  having an appropriate length, respectively. The GI plastic optical fiber  54  constitutes an etalon whose transmission wavelength varies depending on the position along directions indicated by arrows in FIG.  9 . In the GI plastic optical fiber  54 , the refractive index decreases from its center toward its periphery. The index distribution approximately takes a quadratic function. Therefore, optical detectors  55   a  of an optical detector array  55  respectively have annular shapes around the center and are arranged at proper intervals. 
     The arrangement of optical detectors  55   a  should meet the following relations to receive desired wavelengths, respectively: 
     
       
         f(x)−f(x+Δx)=Δn 
       
     
     
       
         Δn=NΔλ/(2nd core )/1/(λ+Δλ)/x/d core   
       
     
     
       
         f(x)=n 0 −4n 0 x 2 /d core   2   
       
     
     
       
         Δx=d dp (when N=N pd ), 
       
     
     where N pd  is the number of optical detectors  55   a , Δλ is the wavelength interval between wavelength multiplexed optical signals transmitted through the optical fiber  1  and input into the demultiplexing element of this embodiment, N is the number of the multiplexed wavelengths, d core  is the core diameter of the optical fiber  1 , and d pd  is the interval between optical detectors  55   a . Here, x is an axis in a direction perpendicular to a traveling direction of light, and n 0  is the highest refractive index at a center of the GI optical fiber  54 . 
     The bottom relation shows a refractive-index distribution in the fiber  54  where 0&lt;x&lt;d core /2. The middle relation shows a desired refractive-index difference, Δn, corresponding to each channel of the multiplexed wavelengths. Δx derived from those relations is the intervals between the optical detectors  55   a . Here, the refractive-index distribution is parabolic, so that Δx is determined by x and the detectors  55   a  are not arranged simply equidistantly. 
     The member  54  having such a refractive-index distribution is conventionally available, and the etalon can be readily constructed by appropriately setting the reflective films  52  and  53 . In this case, the member  54  can be readily coupled or fixed to the optical fiber  1  since the member  54  is a cylindrical optical fiber. The operation is the same as that of the above embodiments. 
     Seventh Embodiment 
     A seventh embodiment of the present invention is shown in FIGS. 10A and 10B. In FIG. 10A, showing a cross section of a plastic optical fiber  1  taken along its optical axis, reflective films  62  and  63  are provided on opposite end surfaces of an optical member  64  consisting of mutually-complementary optical materials having different refractive indices n 1  and n 2 , respectively. 
     The optical member  64  is formed by bringing two optical materials with complementary stepwise surfaces into contact with each other. A stepwise interface is formed in the optical member  64 , as illustrated in FIGS. 10A and 10B. The reflective films  62  and  63  are provided on flat surfaces of the two optical materials. In such a structure, optical lengths between the opposite reflective films  62  and  63  (i.e., sums (n 1 L 1 +n 2 L 2 ) of products between refractive indices n 1  and n 2  and physical lengths L 1 (x) and L 2 (x)) can be varied depending on the position (x). 
     A width (Δ1) in the optical-axial direction of each step in the stepwise refractive-index distribution can be represented as follows: 
     
       
         Δ1(x)=[(N−1)Δλ]/[2n(λ+Δλ)]×(n 1 L 1 −n 2 L 2 )/(n 1 −n 2 ), 
       
     
     where Δλ is the wavelength interval between wavelength multiplexed optical signals transmitted through the optical fiber  1  and input into the demultiplexing element of this embodiment, and N is the number of the multiplexed wavelengths. Here, n 1  and n 2  are refractive indices of the two optical materials (n 1 &gt;n 2 ), L 1  and L 2  are physical lengths of the optical materials having different refractive indices n 1  and n 2 , respectively, and n=(n 1 L 1 −n 2 L 2 )/(L 1 −L 2 ). 
     Since such a distribution is formed in the optical member  64 , there can be formed an etalon in which optical lengths vary depending on places corresponding to the respective detectors  65   a - 65   n  of the detector array  65 . That is, the transmission wavelength in the etalon varies depending on the places corresponding to the respective detectors  65   a - 65   n , so that wavelength multiplexed signals can be demultiplexed and received by the respective optical detectors  65   a - 65   n.    
     In this embodiment, L 2 (x) monotonously increases in the x-axial direction. The refractive-index distribution of the etalon can take any form if only a desired wavelength can be received by each optical detector, as described in the fifth embodiment. The operation is the same as that of the above embodiments. 
     Eighth Embodiment 
     An eighth embodiment of the present invention is shown in FIG.  11 . In FIG. 11, showing a cross section of a plastic optical fiber  71  taken along its optical axis, a diffraction grating  72  is formed on an end surface of the optical fiber  71 , and an optical detector array  75  is provided on an end portion of the fiber  71 . The diffraction grating  72  is formed on the end surface of the optical fiber  71  by using photolithographic techniques or the like. Its angle (an angle of the grating  72  relative to a light traveling direction of the optical fiber  71 ), its pitch and so forth are set such that wavelength multiplexed optical signals are diffracted toward a side location of the fiber  71  on which the detector array  75  is provided. 
     The operation is as follows. When wavelength multiplexed optical signals are transmitted through the optical fiber  71  and reach the end portion of the fiber  71 , respective signals are diffracted by the diffraction grating  72  in different directions according to their wavelengths and received by respective optical detectors  75   a - 75   n  of the detector array  75  to be converted to electric signals. 
     Thus, this embodiment employs wavelength dispersion characteristics of the diffraction grating whose reflection angle of light depends on its wavelength. Since the detector array  75  is provided on the side surface of the fiber  71 , it is fixed along a cylindrical surface of the fiber when the fiber  71  has such a profile. Where the cross section of the fiber  71  is square, the detector array an be readily fixed to a flat side surface of the fiber  71  opposed to the diffraction grating  72 . Other structures and the operation are the same as those of the above embodiments. 
     Ninth Embodiment 
     A ninth embodiment of the present invention is shown in FIG.  12 . In the ninth embodiment, a lens  77  is used to condense the diffracted light from the diffraction grating  72  onto the optical detector array  75 . Thus, wavelength separation can be performed by the diffraction grating  72  more assuredly and effectively. The lens  77  is fixed to a cylindrical support jig  78 , for example. The lens  77  and the detector array  75  can be integrated by molding. In this case, the integrated structure can be readily fixed to the side surface of the optical fiber  71 . 
     Tenth Embodiment 
     A tenth embodiment of the present invention is shown in FIG.  13 . In FIG. 13, showing a cross section of a plastic optical fiber  81  taken along its optical axis, a reflective surface  85  is formed as a 45° slantingly-cut surface of the optical fiber  81 . An optical band-pass filter array  84  with different transmission wavelength bands (λ 1 , λ 2 , λ 3 ) is provided on a side surface of the optical fiber  81 , and an optical detector array  85  is provided on the filter array  84 . Respective flters of the band-pass filter array  84  are formed such that they can receive reflected light from the reflective surface  85  and guide different wavelengths to respective optical detectors  85   a - 85   n  of the detector array  85 . 
     The operation is as follows. When wavelength multiplexed optical signals are transmitted through the optical fiber  81  and reach the reflective surface  85  formed on the end portion of the fiber  81 , respective signals are reflected and guided to the filter array  84 . Light wavelengths are selected by the respective filters of the filter array  84  and guided to the respective detectors  85   a - 85   n  of the detector array  85 . Light received by the respective optical detectors  85   a - 85   n  is converted to electric signals. 
     Also in this embodiment, where the cross section of the fiber  81  is square, the filter array  84  and the detector array  85  can be readily fixed to a flat side surface of the fiber  81  opposed to the reflective surface  85 . In this embodiment, unwanted light of the wavelength multiplexed light is removed by each band-pass filter, so the use efficiency of light is slightly lower compared to demultiplexing methods of the above embodiments. Other structures and the operation are the same as those of the above embodiments. 
     Eleventh Embodiment 
     An eleventh embodiment of the present invention is shown in FIG.  14 . In FIG. 14, showing a cross section of an optical fiber  91  taken along its optical axis, a core-expanding portion  92  is provided on an end face of the optical fiber  91  having a core  91   a , such as a plastic optical fiber with a core diameter of 1 mm. A Fabry-Perot etalon  99  is provided on the core-expanding portion  92 . The etalon  99  is composed of a wedge-shaped optical material  94  of a uniform refractive index and reflective films  96  and  97  provided on opposite non-parallel faces of the optical material  94 , similarly to the first embodiment. An optical detector array  95  is formed on the reflective film  97 . The demultiplexing function of the etalon  99  is as described in the first embodiment. 
     In the core-expanding portion  92 , light transmitted through the optical fiber  91  is expanded to a desired magnitude. For example, where the core diameter of the optical fiber  91  is 1 mm and light is to be expanded to 5 mm, the core-expanding portion  92  needs to have a length of about 20 cm and the most expanded diameter of its end surface connected to the reflective film  96  is about 1 cm. 
     Light transmitted through the optical fiber  91  is gradually expanded in the core-expanding portion  92  (the length of the core-expanding portion  92  needs to have about 20 cm to expand the light naturally and gradually), and the light is converted to a light beam with a diameter of about 5 mm at the end face of the core-expanding portion  92 . The expanded light is input into the etalon  99 , demultiplexed and guided thereby to optical detectors  95   a - 95   n  of the detector array  95 . 
     Also in this embodiment, where cross sections of the fiber  91 , core-expanding portion  92  and etalon  99  are square, the detector array  95  having a similar square cross section can be readily fixed to a flat surface of the reflective film  97 . Other structures and the operation are the same as those of the first embodiment. 
     Twelfth Embodiment 
     A twelfth embodiment of the present invention is shown in FIG.  15 . In FIG. 15, showing a cross section of an optical fiber  91  taken along its optical axis, an optical member  93  having a lens function is inserted into the core-expanding portion  92 . Here, a convex-shaped lens  93  having a refractive index different from that of the core-expanding portion  92  is inserted. In this structure, light can be expanded to a desired magnitude by the core-expanding portion having a shorter length than the structure of FIG.  14 . Other structures and the operation are the same as those of the eleventh embodiment. 
     Thirteenth Embodiment 
     A thirteenth embodiment of the present invention is shown in FIG.  16 . In FIG. 16, showing a cross section of an optical fiber  101  taken along its optical axis, a diffraction grating  103  is formed at an interface between the fiber  101  having a core  101   a  and a core-expanding portion  102 . An optical detector array  105  having optical detectors  105   a - 105   n  is provided on an expanded end face of the core-expanding portion  102 . The core-expanding portion  102  has a refractive index different from that of the core  101   a  in the fiber  101  such that the diffraction grating  103  can be formed therebetween. 
     Wavelength multiplexed signals transmitted through the optical fiber  101  are diffracted by the diffraction grating  103  differently depending on their wavelengths and input into different optical detectors  105   a - 105   n  through the core-expanding portion  102 . Thus, light at each wavelength is converted to an electric signal in the detector. Similarly to the twelfth embodiment, an optical member with a lens function may be inserted into the core-expanding portion  102  to shorten the length of core-expanding portion  102 . Other structures and the operation are the same as those of the above embodiments. 
     Fourteenth Embodiment 
     A fourteenth embodiment of the present invention is shown in FIG.  17 . In FIG. 17, showing a cross section of an optical fiber  111  taken along its optical axis, a deflecting optical switch  113  is provided on an end face of the optical fiber  111  having a core  111   a . A core-expanding portion  112  is provided on the deflecting optical switch  113 . An optical detector array  115  having optical detectors  115   a - 115   n  is formed on an expanded end face of the core-expanding portion  112 . The deflecting optical switch  113  has, for example, a structure in which light is deflected to a varying direction depending on its wavelength due to the acousto-optical effect, or light is scan-deflected with time. 
     In this embodiment, time division multiplexed optical signals transmitted through the optical fiber  111  are deflected to different directions per channel by the deflecting optical switch  113 . Each deflected light signal is input into a desired detector  115   a - 115   n  of the detector array  115 . Other structures and the operation are the same as those of the eleventh embodiment. 
     Fifteenth Embodiment 
     A fifteenth embodiment of the present invention is shown in FIG.  18 . In FIG. 18, showing a cross section of an optical fiber  121  taken along its optical axis, a demultiplexing portion  122  is connected to the optical fiber  121 . Diffraction gratings  123   a - 123   c  are formed on surfaces of the demultiplexing portion  122 . The diffraction grating  123   a  is formed at an interface between the optical fiber  121  and the demultiplexing portion  122 . An optical detector array  125  with optical detectors  125   a - 125   n  is provided on the surface of the demultiplexing portion  122 . 
     Wavelength multiplexed signals transmitted through the optical fiber  121  are received and diffracted by the diffraction grating  123   a , and the diffracted light is guided to the diffraction grating  123   b . Then, light diffracted by the diffraction grating  123   b  is guided to the diffraction grating  123   c  and diffracted thereby to be guided to the detector array  125 . Here, the wavelength multiplexed optical signals are successively diffracted by the plural diffraction gratings  123   a - 123   c  and input into the detector array  125 , so that demultiplexing can be performed with a high wavelength resolving power. Other structures and the operation are the same as those of the thirteenth embodiment. 
     Sixteenth Embodiment 
     A sixteenth embodiment of the present invention is shown in FIG.  19 . In FIG. 19, showing a cross section of an optical fiber  1  taken along its optical axis, a wedge-shaped Fabry-Perot etalon  9 , an optical detector array  5 , and a signal receiving circuit  134  are provided on an end surface of the optical fiber  1 . The end portion of the fiber  1 , etalon  9 , detector array  5  and signal receiving circuit  134  are molded with resin  135 . The wedge-shaped Fabry-Perot etalon  9  can have a structure as described in the first to seventh embodiments. The molding resin  135  is formed such that the etalon  9 , detector array  5  and signal receiving circuit  134  are encompassed thereby. Electrode terminals  136  connect the signal receiving circuit  134  to an external unit through the resin  135  such that an electric power can be supplied to an electric power source of the signal receiving circuit  134  and electric signals converted from received optical signals can be taken out. 
     In such a structure, optical components (herein the wedge-shaped Fabry-Perot etalon  9  and the detector array  5  such as a pin photodiode) and electric components (herein the signal receiving circuit  134 ) can be readily fixed to the end face of the optical fiber  1  under an optical-axial alignment condition. Thus, axial deviations between the components can be readily prevented. 
     An electric power is supplied to the signal receiving circuit  134  through a few (for example, two) of the electrode terminals  136 . The signal receiving circuit  134  processes the signals detected by the optical detector array  5  and outputs the processed signals to other electrode terminals  136 . The operation of the optical portion is the same as that of the first embodiment. 
     Seventeenth Embodiment 
     A seventeenth embodiment of the present invention is shown in FIG.  20 . While the wedge-shaped Fabry-Perot etalon  9  and the detector array  5  are stacked in a mutual contact state on the end face of the optical fiber  1  and molded with the resin  135  in FIG. 19, in the seventeenth embodiment a portion  146  of the optical fiber  1  (here a clad  1   b  at an end portion of the optical fiber  1 ) is left extending in its axial direction and optical and electric components are placed on the portion  146  and molded with resin  145 . In this embodiment, the optical and electric components  5 ,  9  and  134  can be set and fixed by molding while the optical fiber  1  being layed, so its fabrication can be facilitated and its yield can be improved. Here, where the fiber  1  has a square cross section, the clad  146  on one side can be left and the optical and electric components  5 ,  9  and  134  can be placed and moled thereon. Hence the fabrication can be further facilitated. 
     An electric power is supplied to the signal receiving circuit  134  through a few (for example, two) of the electrode terminals  136 . The signal receiving circuit  134  processes the signals detected by the optical detector array  5  and outputs the processed signals to other electrode terminals  136 . The operation of the optical portion is the same as that of the first embodiment. 
     Eighteenth Embodiment 
     An eighteenth embodiment of the present invention is shown in FIG.  21 . In FIG. 21, showing a cross section of an optical fiber  1 , a portion of a core  1   a  at an end portion of the fiber  1  is removed, for example, by etching. Optical detector array  5 , etalon  9 , and signal receiving circuit  134  are put in an end portion  156  of a remaining clad  1   b  of the fiber  1  and molded with resin  155 . In this embodiment, the optical and electric components  5 ,  9  and  134  can be further readily fixed, and the amount of the resin  155  to be used for molding can be reduced. 
     In the sixteenth to eighteenth embodiments, though demultiplexing means, such as the Fabry-Perot etalon, and the detector array are used as the optical component, the optical component is not limited thereto and other optical components can also be used for the mold structure. Further, while the optical and electric components are integrally molded in the above embodiments, only optical components are molded and the electric circuit may be placed outside the mold structure, for example. 
     In several embodiments discussed above, the optical fiber having a square or rectangular cross section is referred to, so such kind of optical fibers will be described in more detail hereinafter. 
     Where the optical fiber has a square cross section consisting of a square core  201   a  and a clad  201   b  encompassing the square core  201   a , as illustrated in FIG. 22A, a detector array  204  also preferably has a corresponding square cross section and consists of plural rectangular optical detectors  204   a - 204   n , as illustrated in FIG.  22 B. This is also true with demultiplexing means, such as a Fabry-Perot etalon. In this structure, the shape of the core  201   a  approximately fits with the total shape of the detectors  204   a - 204   n , so the optical fiber can be optically coupled to the respective detectors  204   a - 204   n  effectively. 
     In the case of a plastic optical fiber, the fiber is once shaped into a configuration having a circular cross section and then re-shaped into a square cross section by appropriately applying heat thereto. When the optical fiber has the square cross section over the entire length, the fiber can be used as a polarization conservative optical fiber which maintains the polarization state of transmitted light. 
     A core-expanding portion  205  as illustrated in FIG. 23A (FIG. 23A shows a cross section at a certain location of the core-expanding portion  205 ) can be fixed to the end face of the fiber with a square cross section. The core of FIG. 23A is gradually expanded with respect to up and down directions while its rectangular cross section is maintained. In this case, a detector array to be fixed to the end surface of the core-expanding portion  205  has a structure consisting of optical detectors  214   a , . . . ,  214   k , . . . ,  214   n , as illustrated in FIG.  23 B. This is also true with demultiplexing means, such as a Fabry-Perot etalon. 
     FIGS. 24A and 24B illustrate an optical fiber  221  which includes three independent rectangular-shaped cores  221   a - 223   a  and a clad  221   b  encompassing these cores  221   a - 223   a . In the case of the separate cores over the entire length of the fiber, parallel transmission of plural channels can be readily achieved. Here, light sources such as LEDs and LDs are coupled to the respective cores  221   a - 223   a  by butt-coupling on a transmitter side, and optical detectors are formed corresponding to the respective cores  221   a - 223   a  on a receiver side. Thus, plural optical signals can be stably transmitted and received with little crosstalk. This is also true with demultiplexing means, such as a Fabry-Perot etalon. 
     FIG. 25 illustrates an optical fiber  231  whose core  231   a  is divided into three rectangular-shaped cores  232   a - 234   a  at its end portion and which includes a clad  231   b . Where the optical fiber  231  has a relatively large core size (for example, about 1 mm) which permits a large number of modes, a change of S/N ratio due to the division can be greatly lowered. Such a structure can be used as a star coupler which divides a single input into plural outputs, for example. 
     FIG. 26 illustrates another example of an optical fiber having a core of a square cross section. In FIG. 26, reference numeral  241  denotes a first optical fiber, reference numeral  242  denotes a second optical fiber, reference numeral  243  denotes a coupling portion, reference numeral  244  denotes a clad, and reference numeral  245  denotes a core. The optical fibers  241  and  242  are respectively plastic optical fibers with a core size of about 1 mm, for example. Cores  245  of the first and second optical fibers  241  and  242  are coupled through a thin clad  244  therebetween at the coupling portion  243 . The core  245  has a square cross section at least at the coupling portion  243 . The coupling portion can be readily formed due to the square shape of the core  245 . 
     In FIG. 26, the directional coupler is built with a coupling length of 1 and an interval of d. The coupling length is determined by 1=π/(β e −β o ). However, where the optical fibers  241  and  242  are multi-mode fibers, there exist even-mode propagation constant (β e ) and odd-mode propagation constant (β o ) for each wavelength and hence appropriate coupling lengths vary depending on the wavelengths. Therefore, light can be effectively coupled from one optical fiber to another by setting the coupling length to a mean value of the coupling lengths for respective modes, though light power will not be completely transferred even in this case. Where light intensities vary for respective modes in the multi-mode fiber, the transfer of light power increases when the coupling length is determined from the propagation constants for even and odd modes in an area where there are many modes having large intensities. 
     Except as otherwise disclosed herein, the various components shown in outline or block form in any of FIGS. 1A-26 are individually well known in the optical fiber devices, optical detector devices, demultiplexing devices and electric circuits, and their internal construction and operation are not described herein. 
     While the present invention has been described with respect to what are presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. The present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.