Abstract:
It is the object of the invention to integrally conform functions of devices of optical fiber types with those of SiO 2  waveguides, miniaturize optical devices and provide optical waveguide devices with various optical functions. A SiO 2  waveguide, which is composed of a SiO 2  waveguide clad with low refractive index and a SiO 2  waveguide core with high refractive index, is formed on a Si substrate. The core and the clad on an area, where the direction of the core coincides with the orientation of crystallization of a Si substrate, are removed. A V-groove is formed along the orientation of crystallization of the exposed Si substrate. Both the end surfaces of an optical fiber grating, which is fitted in the V-groove, face those of the remaining SiO 2  waveguide. The cores of the SiO 2  waveguides and the optical fiber grating are optically coupled with each other.

Description:
FIELD OF THE INVENTION 
     The invention relates to an optical waveguide device for optical communication, and especially to an optical waveguide device, in which an optical device of an optical fiber type is combined with optical waveguide circuits. 
     BACKGROUND OF THE INVENTION 
     An optical fiber grating (a fiber grating, hereinafter) has a property of reflecting optical signal with a particular wavelength propagating therethrough, and serves as an optical filter. 
     Conventionally, when the fiber grating is used in an optical circuitry, the fiber grating is connected with individual optical devices, such as a directional coupler, a Y-branch and etc., the fiber grating is connected with them via an optical fiber in most cases. Among the aforementioned optical devices, the Y-branch and the directional coupler can be fabricated from SiO 2  waveguides, but the characteristic of the fiber grating is better than that of a SiO 2  waveguide grating. 
     Moreover, the Er (erbium)-doped optical fiber is about several tens meters long and wound round a reel in most cases. The directional coupler or the Y-branch is connected with one end of the Er-doped fiber, and a pumping light with a wavelength of 1.48 μm or 0.98 μm is supplied thereto. An optical signal with the wavelength in 1.55 μm band is supplied from the other end of the Er-doped optical fiber. The aforementioned devices are connected with the end of the Er-doped fiber by means of arc fusion. 
     In the aforementioned devices, the directional coupler and the Y-branch can be fabricated from the SiO 2  waveguide, but the characteristic of the Er-doped fiber is better than that of an Er-doped SiO 2  waveguide. 
     In case that the fiber grating or the Er-doped fiber is used as an optical device, since they are connected with the directional coupler or the Y-branch by means of arc-fusion, fused portions must be reinforced after the work of arc fusion is over and become voluminous, hence miniaturization of this portion is difficult. 
     Moreover, the disadvantage of the optical device, such as the fiber grating, which suppresses the optical signal with a particular wavelength, is that its characteristic is changed by bending and stretching. On the other hand, in the SiO 2  waveguide, functions of splitting an optical signal and demultiplexing plural optical signals have been already completed, and technology for integrally realizing these functions have been established. However, in the SiO 2  waveguide, functions of amplifying and isolating the optical signal have not yet been realized. 
     As mentioned in the above, the devices of the fiber type and those of the SiO 2  type respectively have their own advantages, and are connected with each other via an optical fiber. 
     Accordingly, a troublesome job concerning with dealing with the remaining optical fiber is inevitable, and miniaturization of the connected portion becomes difficult. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide an optical filter of an optical waveguide type to overcome disadvantages of conventional technology, and integrally conform the functions of devices of an optical fiber type with those of the devices of SiO 2  waveguide type. 
     It is a further object of the invention to provide an optical isolator of an optical waveguide type to overcome disadvantages of conventional technology, and integrally conform the functions of devices of an optical fiber type with those of the devices of SiO 2  waveguide type. 
     It is a still further object of the invention to provide an optical amplifier of an optical waveguide type to overcome disadvantages of conventional technology, and integrally conform the functions of devices of an optical fiber type with those of the devices of SiO 2  waveguide type, 
     According to the first feature of the invention, an optical waveguide device comprises: 
     an optical waveguide formed on a substrate, 
     a Y-branch of an optical waveguide type formed on the substrate, and, 
     an optical fiber grating with a predetermined length set between the optical waveguide and the Y-branch, 
     wherein both end surfaces of the optical fiber grating respectively face an end surfaces of a core of the optical waveguide and an end surface of a core of a trunk part of the Y-branch. 
     According to the second feature of the invention, an optical waveguide device comprises: 
     two optical waveguide respectively formed on both side ends of a substrate, 
     two multi-mode optical fibers of graded index type with equal predetermined lengths set in series between the two optical waveguides, and 
     an optical isolator inserted between the two multi-mode optical fibers of graded index type, 
     wherein cores of the optical waveguides, cores of the two multi-mode optical fibers of graded index type and the optical isolator are optically and successively connected. 
     According to the third feature of the invention, an optical waveguide device comprises: 
     plural rare earth metal-doped optical fibers with a predetermined length arranged on a substrate, 
     plural optical waveguide circuits respectively comprising optical band pass filters, which alternately connect ends of the plural rare earth metal doped optical fibers with those of the adjacent plural rare earth metal doped optical fibers to form a zigzag path for an optical signal supplied from a port of an input optical waveguide. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail in conjunction with appended drawings, wherein: 
     FIG. 1 is a perspective view of the first preferred embodiment of the invention, in which optical waveguides are combined with an optical fiber grating, 
     FIG. 2 is a perspective view of the second preferred embodiment, in which an optical isolator is inserted between short pieces of multi-mode optical fibers of graded index type, and 
     FIG. 3 is a perspective view of the third preferred embodiment of the invention, in which Er-doped optical fibers of short lengths are inserted between optical waveguides. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, three preferred embodiments of the invention will be explained referring to appended drawings. 
     First, the first preferred embodiment will be explained. 
     FIG. 1 shows a perspective view for showing the first preferred embodiment of the invention, which combines a fiber grating with SiO 2  waveguides. 
     A SiO 2  waveguide is formed on a Si substrate 1, then the central part of the SiO 2  waveguide is removed, and finally a V-groove 2 is formed on the exposed Si substrate. Moreover, an optical fiber grating 6 is fitted in the V-groove 2. 
     The process of fabrication of the structure shown in FIG. 1 comprises the steps of: 
     depositing a mask for forming a V-groove on the central part of a Si substrate, 
     forming the SiO 2  waveguide on the Si substrate, 
     removing the SiO 2  waveguide at the central part of the Si substrate, on which the V-groove is to be formed, 
     forming the V-groove by etching, and 
     fitting an optical fiber grating in the V-groove. 
     First, a mask pattern necessary for etching the V-groove is formed on the Si substrate. The V-groove 2 is formed by wet etching. 
     The position of the mask necessary for forming V-groove 2 should be determined in consideration of that of the core of the SiO 2  waveguide, which will be formed in a later process. 
     The direction of the V-groove 2 should coincide with that of orientation of crystallization of Si, which constitutes the Si substrate. 
     The material of the mask necessary for forming the V-groove 2 is a heat-resisting metal withstanding temperature of nearly 1000° C., which can be formed by either spattering or evaporation. 
     Next, the SiO 2  waveguide is formed on the Si substrate 1. In this step, SiO 2  with low refractive index is deposited on the Si substrate 1, and SiO 2  with high refractive index is subsequently deposited thereon. A part of SiO 2  with high refractive index will be formed into the core of the SiO 2  waveguide in a later process. As the method for depositing SiO 2  layer, that of TEOS-CVD, flame hydrolysis deposit or spattering can be adopted. 
     In order to reform the deposited SiO 2  with high refractive index into the core of the SiO 2  waveguide, photolithography technology is adopted. That is to say, on the top surface of the SiO 2  layer with high refractive index, the mask is left behind by photolithography over a region, where the core of the SiO 2  waveguide is to be formed. In the area, which is not covered by the mask, the SiO 2  layer with high refractive index is removed by dry etching, such as reactive ion etching (RIE). The SiO 2  with low refractive index is again deposited in order to cover the SiO 2  region with high refractive index, which has been left behind in the aforementioned dry etching process. The aforementioned techniques are commonly used for fabricating the SiO 2  waveguide. 
     Next, the SiO 2  waveguide formed on the region, where the V-groove 2 is to be formed, is removed. First, the part of the SiO 2  waveguide, which is to be left behind, is masked by photolithography. Subsequently, the SiO 2  waveguide, which is not covered by the mask, is removed by reactive ion etching, and the surface of the Si substrate 1 is exposed. 
     Next, the V-groove 2 is formed by wet etching utilizing anisotropic property of Si. Explaining concretely, the surface of the Si substrate 1 is covered with a mask by photolithography, which withstand etchant for Si, such as water solution of KOH, except a region, where the V-groove 2 is to be formed. A thermally oxidized layer of Si can be used as a mask, but the metallic mask, which withstands high temperature, is used in the embodiment. As mentioned in the above, the mask has been formed before the SiO 2  waveguide is formed. Next, the V-groove 2 is etched by KOH solution with concentration of about 35 weight percent. The width of the V-groove 2 is determined in consideration of the height of SiO 2  waveguide core above the Si substrate 1. In this example of the embodiment, the SiO 2  waveguide core 15 μm high above the Si substrate 1, the diameter of the fiber grating 6 is 125 μm and the width of the V-groove 2 is 132 μm. 
     In boundary regions between the end surfaces of the Si substrate 1 and both the ends of the V-groove 2, rectangular grooves 3 are so formed by a dicing saw that they are perpendicular to the V-groove 2 in order to obtain their vertically processed end surfaces and remove unnecessary portions. 
     The fiber grating 6, which is fitted in the V-groove 2 and has a diameter of 125 μm, reflects 99% of an optical signal with a wavelength of 1.55 μm. 
     A nearly white light, which is supplied to the port of the SiO 2  waveguide core 5A in the optical device shown in FIG. 1, arrives at the input end of the fiber grating 6 via the Y-branch. A part of the incident light, which has wavelength of 1.55 μm, is reflected by the fiber grating. 50% of the reflected light returns to the port of the SiO 2  waveguide core 5A, and remaining 50% of that enters the port of a SiO 2  waveguide core 5B. Moreover, the other part of the incident light, which propagates through the fiber grating 6, enters the port of a SiO 2  waveguide core 5C. 
     Next, the second preferred embodiment of the invention will be explained. 
     FIG. 2 is a perspective view of the second preferred embodiment, in which an optical isolator is combined with optical waveguides. By a similar method to that in the case of FIG. 1, the SiO 2  waveguides are formed near both the ends of the V-groove 2. In other words, the central part of the SiO 2  waveguide formed by a Si substrate 1 is removed by etching, the Si substrate 1 is exposed and the V-groove 2 is formed thereon. 
     Next, in order to so form the end surfaces of the SiO 2  waveguide clads 4 and the SiO 2  waveguide cores 5 that they are smooth and perpendicular to the Si substrate 1, they are processed by a dicing saw. 
     A multi-mode optical fiber of graded index type is suitably divided into two parts 7 and 8, and they are respectively fitted into the V-groove 2. In this case, the end surfaces of the multi-mode fibers are so fixed that they face the end surfaces of the SiO 2  waveguide. An interval between the multi-mode fibers 7 and 8 is several mm. The refractive index of the multi-mode fibers 7 and 8 is represented by a decreasing parabolic function in the radial direction, and they serve as collimator lenses by suitably selecting their lengths. Accordingly, a ray of light focused on the optical isolator 9 by the multi-mode fiber of graded index type 7 is again focused on the input end of the SiO 2  waveguide core 5 by the multi-mode optical fiber of graded index type 8, and an optical loss therebetween can be decreased. 
     In this embodiment of the invention, an optical isolator 9 is allocated between the multi-mode fibers 7 and 8. 
     Next, the third preferred embodiments will be explained. 
     FIG. 3 is a perspective view for showing the third preferred embodiment, which is fabricated by combining an Er-doped optical fiber with SiO 2  waveguide. 
     SiO 2  waveguides are formed near both the ends of V-grooves 2 by a method similar to that used in the case of FIG. 1. That is to say, the central parts of the SiO 2  waveguides formed on a Si substrate 1 are removed by etching, and the Si substrate 1 is exposed, and a V-grooves 2 are formed thereon. 
     Next, in order to so form the end surfaces of the SiO 2  waveguide cores 5 that they are smooth and perpendicular to the Si substrate 1, they are processed by a dicing saw. 
     The SiO 2  waveguides near both the ends of the V-grooves 2 are formed in parallel with the longitudinal direction of the V-grooves 2 at a pitch of 130 μm. The SiO 2  waveguide becomes closer to the neighboring one, as they are remote from the V-grooves 2, and an optical power propagating through the SiO 2  waveguide shifts to the neighboring one. The SiO 2  waveguides are so designed that 50% of the optical power propagating through the SiO 2  waveguide shifts to the neighboring one, when they arrive at their end surfaces. 
     Multiple dielectric layer filters 11-12 are stuck to the end surfaces of the SiO 2  waveguides and reflect only the optical signals of a predetermined wavelength. The optical signal is reflected by the end surface of the SiO 2  waveguide and propagates through the same SiO 2  waveguide in the opposite direction. The optical power propagating in the opposite direction subsequently shifts to the neighboring SiO 2  waveguide, till 100% of the initial optical powers shifts thereto. 
     The neighboring SiO 2  waveguides are so designed that the optical signal propagates along a zigzag path and passes through all of the Er-doped fibers arranged on the V-grooves 2. 
     That is to say, the incident optical signal is supplied to the Er-doped fiber 10 via the SiO 2  waveguide core. The optical signal transmitted through the Er-dope fiber 10 is supplied to the other SiO 2  waveguide core, reflected by the multiple dielectric layer filters 11 and 12 and supplied to the next Er-doped optical fiber 10. The optical signal passes through the plural Er-doped fibers 10, repeatedly going and returning, and is emitted from the output end of the SiO 2  waveguide. 
     The multiple dielectric layer filters 11 and 12 reflects only the optical signal with a particular wavelength, and consequently suppresses degrade of S/N ratio, which is caused by spontaneous emission in the step of light amplification. 
     As mentioned in the above, the effects of the invention can be summarized as follows. 
     (1) The optical device can be miniaturized by optically combining the optical fiber core with the SiO 2  waveguide core. 
     (2) The optical fiber can be realized by integrally combining the filter characteristic of the optical fiber grating with Y-branch of the SiO 2  waveguide type. 
     (3) If the two pieces of the multi-mode optical fiber of graded index type are used as the collimator lenses, various optical functions can be realized by setting the optical isolator, the optical band pass filter and a wavelength plate between the aforementioned collimator lenses. 
     (4) The optical amplifier can be realized by integrally combining the light-amplifying function of the Er-doped fiber with the directional coupler of the SiO 2  waveguides type. 
     Although the invention has been described with respect to specific embodiment for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modification and alternative constructions that may be occurred to one skilled in the art which fairly fall within the basic teaching here is set forth.