Diffracting optical apparatus

Abstract: A diffracting device consists of a substrate made of LiTaO₃, an optical waveguide extending in a central upper side of the substrate for transmitting 860 nm wavelength coherent light radiated from a semiconductor laser, a plurality of gratings periodically arranged on the optical waveguide at regular intervals, and a covering layer arranged on the gratings and the optical waveguide. The optical waveguide is formed according to a proton-exchange process. The gratings are made of photoresist material having a refractive index N1=1.5 and a high workability, and the gratings are uniformly formed according to an interference-exposure process. Therefore, the photoresist material is patterned without damaging the optical waveguide with reactive ions. The covering layer is made of Ta₂O₅ having a refractive index N2=2.0. Therefore, periodic change in the refractive index is generated in a periodic structure of the gratings and the covering layer, and a part of the coherent light is reflected by the periodic structure to fix the wavelength of the coherent light radiated from the semiconductor laser.

Description: The present invention relates to a diffracting device having a distributed Bragg reflector, a method of manufacturing the diffracted device, a wavelength changing device including the diffracting device, a laser beam generating apparatus in which a laser beam having a fixed wavelength is generated with the diffracting device and an optical information processing apparatus using the diffracting device. A diffracting device is important to be utilized for not only a device having an optical waveguide but also a light integrated circuit. In cases where a plurality of gratings are periodically arranged in an optical waveguide to manufacture a diffracting device, light propagated through the optical waveguide is controlled by the gratings. For example, in cases where the gratings periodically arranged in the optical waveguide act as a distributed Bragg reflector, coherent light having a particular wavelength is selectively reflected in the optical waveguide, and the coherent light reflected is propagated through the optical waveguide in the opposite direction. FIRST PREVIOUSLY PROPOSED ART: A conventional diffracting device is described with reference to Fig 1. Fig. 1 is a diagonal view of a conventional diffracting device having a distributed Bragg reflector. As shown in Fig. 1, a conventional diffracting device 11 consists of a LiNbO 3 substrate 12, a plurality of gratings 13 periodically arranged in series in a central surface of the substrate 12 at regular intervals Λ1, and a Ti diffused optical waveguide 14 extending from one side of the substrate 12 to the other side through the gratings 13. In the above configuration, light beams having various wavelengths are radiated to an incident surface 14a positioned at one side of the optical waveguide 14, and a particular light beam having a particular wavelength is reflected by the gratings 13 because periodic change in a refractive index of the optical waveguide 14 is formed by the gratings 14 periodically arranged. That is, the gratings 13 act as a distributed Bragg reflector. Therefore, the particular light beam is output from the incident surface 14a of the optical waveguide 14, and remaining light beams except the particular light beam are output from an output surface 14b positioned at the other side of the optical waveguide 14. Next, a conventional manufacturing method of the conventional diffracting device 11 is described with reference to Figs. 2A to 2D. The method is performed with two superimposed masks (J. SOCHTIG, "Ti:LiNbO 3 Stripe Waveguide Bragg Reflector Grating", Electronics Letters, Vol.24, No.14, p.844-845 (1988)). As shown in Fig. 2A, after the optical waveguide 14 is formed by diffusing Ti into a central surface region of the substrate 12, a thin Ti film 15 is deposited on the substrate 12 and the optical waveguide 14. The thin Ti film 15 is utilized as a first superimposed mask. Thereafter, a photoresist 16 is spin coated on the Ti film 15. The photoresist 16 is utilized as a second superimposed mask. Thereafter, as shown in Fig. 2B, the photoresist 16 is exposed to interference light according to an interference-exposure process, and the photoresist 16 exposed is developed to remove exposed areas of the photoresist 16. Therefore, a periodic grating pattern is transferred to the photoresist 16. Thereafter, as shown in Fig. 2C, the Ti film 15 is periodically etched at regular intervals Λ1 by reactive ions generated in an atmosphere of CCl 2 F 2 gas according to a reactive ion etching to transfer the periodic grating pattern of the photoresist 16 to the Ti film 15. Thereafter, as shown in Fig. 2D, the patterned film 15 is used as a mask, and the LiNbO 3 substrate 12 is etched at the regular intervals Λ1 by reactive ions generated in an atmosphere of CF 4 , Ar, and N 2 according to the reactive ion etching. Therefore, the gratings 13 are periodically formed in surface portions of the substrate 12 at the regular intervals Λ1. Thereafter, both sides of the optical waveguide 14 are polished. Fig. 3 graphically shows transmitting and reflecting characteristics of the diffracting device 11. As shown in Fig. 3, when light beams having wavelengths of 1.5 µm band are radiated from a light emitting diode and are coupled to the optical waveguide 14 of the diffracting device 11, a particular light beam having a particular wavelength λp which satisfies a Bragg condition is selectively reflected. The Bragg condition is determined by regular intervals of the gratings 13 and the effective refractive index of the grating. SECOND PREVIOUSLY PROPOSED ART: Fig. 4 is a cross-sectional view of another conventional diffracting device. As shown in Fig. 4, another conventional diffracting device 21 consists of a glass substrate 22, an optical waveguide 23 formed in a central surface portion of the substrate 22 according to an ion-exchange process, and a plurality of SiO 2 gratings 24 periodically arranged at regular intervals Λ1=1.2 µm. A total length of the SiO 2 gratings 24 is 10 mm in a propagation direction of the coherent light. A distributed Bragg reflector is formed by a periodic structure composed of the SiO 2 gratings 24 and spaces between the gratings 24 in cases where a distributed Bragg reflector condition (or DBR condition) Λ1 = mλ/2N is satisfied. Here the symbol Λ1 denotes the regular intervals of the gratings 24, the symbol m is a grating order of the periodic structure, the symbol λ denotes a wavelength of coherent light, and the symbol N denotes an averaged refractive index of the periodic structure. When the wavelength of the coherent light is 1.3 µm, the DBR condition is satisfied to reflect the coherent light in the periodic structure of which the grating order m is equal to 3. In the above configuration, coherent light converged at an incident end facet 23a transmits through the optical waveguide 23. In this case, a part of the coherent light is distributed off the optical waveguide 23, so that the coherent light distributed off the optical waveguide 23 is reflected by the gratings 24. Next, a manufacturing method of the diffracting device 21 is described. After the optical waveguide 23 is formed in the substrate 22, a SiO 2 film is deposited on the optical waveguide 23 and the substrate 22. Thereafter, a photoresist film is coated on the SiO 2 film. Thereafter, grating pattern areas of the photoresist is selectively exposed to ultraviolet radiation according to a conventional interference-exposure process, and the photoresist is developed to remove the grating pattern areas of the photoresist. Therefore, a grating pattern is transferred to the photoresist film. Thereafter, the SiO 2 film is etched by reactive ions according to a dry etching while the photoresist film is utilized as a mask. Therefore, the grating pattern is transferred to the SiO 2 film, and the gratings 24 made of SiO 2 are formed on the optical waveguide 23. When 1.3 µm wavelength coherent light is coupled to the optical waveguide 23, 5 % of the coherent light is reflected by the SiO 2 gratings 24. However, because the substrate 12 is made of a hard material LiNbO 3 , complicated processes are required to directly etch the substrate 12 in the conventional diffracting device 11. Also, it is difficult to etch the gratings 13 made of the hard material by a predetermined depth. Therefore, the reproducibility of the apparatus 11 deteriorates, and the gratings 13 are often excessively etched. Also, the surfaces of the gratings 13 become rough because of the radiation of the reactive ions. Therefore, light beams transmitting through the optical waveguide 14 are increasingly scattered. In the same manner, because the gratings 24 on the substrate 22 are made of a hard material SiO 2 , complicated processes are required to form the gratings 24 according to an etching process in the conventional diffracting device 21. Also, it is difficult to etch the gratings 24 without erroneously etching the optical waveguide 23 according to a dry etching process. Therefore, the reproducibility of the apparatus 21 deteriorates, and the gratings 24 are often excessively etched to etch the optical waveguide 23. As a result, the surfaces of the optical waveguide 24 become rough so that the coherent light is increasingly scattered. Also, it is difficult to etch material having a large refractive index and a large transmission coefficient because an etching rate of those materials is very low in general. Therefore, it is troublesome to deeply form the gratings 13, 24. As a result, it is difficult to reflect the light with high reflecting efficiency. Also, because the gratings 13, 24 are formed according to the complicated processes in which a plurality of pattern transferring processes are performed, the unevenness of the periodic pattern in the gratings 13, 24 is increased. Therefore, as shown in Fig. 3, though the reflection of the light beams theoretically occurs at a single particular wavelength λp, the reflection of the light beams actually occurs in a wide wavelength range. In other words, the condition that the gratings 13, 24 function as the distributed Bragg reflector deteriorates because of the complicated processes. Also, because the unevenness of the periodic pattern in the gratings 13, 24 is increased and because the light transmitting through the optical waveguides 14, 23 is scattered by the roughness of the gratings 13 and the optical waveguide 23, a transmission loss of the fundamental waves is increased. Therefore, the intensity of the light is lowered, and a diffraction efficiency of the gratings 13, 24 is lowered. Also, because the position of the gratings 13, 24 is limited near to the surfaces of the substrates 12, 22, the intensity of the light reflected by the gratings 13, 24 is limited. Therefore, it is difficult to reflect the light with high reflecting efficiency unless the length of the gratings 13, 24 extending in a propagation direction is extremely lengthened to increase the number of gratings 13, 24. A wavelength changing device having an optical waveguide has been proposed. The optical waveguide is provided with alternate rows of non-inverted and inverted polarization layers to change fundamental waves transmitting through the optical waveguide to second harmonic waves. The inverted polarization layers are formed by compulsorily inverting the non-linear polarization of ferroelectric substance. The wavelength changing device is utilized for a small-sized shorter wavelength laser beam generating apparatus because fundamental waves radiated from a semiconductor laser are changed to second harmonic waves such as a green or blue light. Therefore, the wavelength changing device is useful in a printing operation, an optical information processing, an optical applied measuring control field, and an optical communication field. The wavelength change in the wavelength changing device can be performed with high efficiency because fundamental waves radiated from a semiconductor laser are changed to second harmonic waves in the alternate rows of non-inverted and inverted polarization layers. Also, because the wavelength of the fundamental waves changed to the second harmonic waves depends on regular intervals of the alternate rows, the wavelength of the second harmonic waves obtained in the wavelength changing device can be arbitrarily changed. However, because the regular intervals of the alternate rows in the wavelength changing device are fixed, the output power of the second harmonic waves considerably fluctuates when the wavelength of the fundamental waves radiated from a semiconductor laser fluctuates. For example, the change of wavelength in a shorter wavelength laser beam generating apparatus has been proposed (K. Yamamoto et. al, "Milliwatt-Order Blue-light Generation in a Periodically domain-Inverted LiTaO 3 waveguide", Optical Letters, Vol.16, No.15, p.1156-1158, (1991)). In the laser beam generating apparatus of Yamamoto, fundamental waves of semiconductor laser beams are changed to second harmonic waves in an optical waveguide having alternate rows of non-inverted and inverted polarization layers according to quasi-phase matching. Fig. 5 is a constitutional view of a conventional shorter wavelength laser beam generating apparatus. As shown in Fig. 5, a conventional shorter wavelength laser beam generating apparatus 31 consists of a semiconductor laser 32, a collimator lens 33 for collimating fundamental waves radiated from the semiconductor laser 32, a λ/2 plate 34 for rotatively polarizing the fundamental waves, a focusing lens 35 having a numerical aperture NA=0.6, and a wavelength converting device 36 having an optical waveguide 37 for changing the fundamental waves converged at an incident end facet 37a to second harmonic waves such as blue light according to the quasi-phase matching. The optical waveguide 37 is provided with alternate rows of non-inverted and inverted polarization layers. The incident end facet 37a and an output end facet 37b of the optical waveguide 37 are coated with antireflection coating to prevent the fundamental waves from being reflected in the incident and output end facets 37a, 37b. In the above configuration, 874 nm wavelength fundamental waves are radiated from the semiconductor laser 32 and are collimated by the collimator lens 34. Thereafter, the fundamental waves are rotatively polarized by the λ/2 plate 34 and are converged at the incident end facet 37a of the optical waveguide 37 by the focusing lens 35. In this case, though the antireflection coating is coated on the incident end facet 37a, approximately 1 % of the fundamental waves are fed back to the semiconductor laser 32 in practical use. Thereafter, blue light consisting of 437 nm wavelength second harmonic waves are radiated from the output end facet 37b of the optical waveguide 37 on condition that a quasi-phase matching condition formulated by an equation Λ2 = λ f /{2*(N2ω-Nω)} is satisfied. Here the symbol Λ2 denotes regular intervals of the alternate rows in the optical waveguide 37, the symbol λ f denotes a wavelength of the fundamental waves, the symbol N2ω denotes an effective refractive index of the non-inverted and inverted polarization layers for the second harmonic waves, and the symbol Nω denotes an effective refractive index of the non-inverted and inverted polarization layers for the fundamental waves. Accordingly, the fundamental waves of such infrared light can be reliably changed to blue light. For example, when the pumping power of the fundamental waves converged at the incident end facet 37a of the optical waveguide 37 is 35 mW, the pumping power of the blue light radiated from the output end facet 37b is 1.1 mW. However, because the blue light is generated by changing the fundamental waves to the second harmonic waves and multiplying the second harmonic waves in the optical waveguide 37 in which the alternated rows of the non-inverted and inverted polarization layers are arranged at regular intervals, a wavelength range of the fundamental waves allowed to obtain the second harmonic waves is only 0.2 nm in the optical waveguide 37. Also, the wavelength of the fundamental waves radiated from the semiconductor laser 32 fluctuates depending on the ambient temperature of the semiconductor laser 32. The fluctuation ratio of the wavelength to the ambient temperature is about 0.2 nm/°C. Therefore, in cases where the ambient temperature of the semiconductor laser 32 varies by 1 °C, the blue light cannot be generated in the optical waveguide 37. In addition to the fluctuation of the ambient temperature, the amplification mode of the fundamental waves radiated from the semiconductor laser 32 varies because approximately 1% of the fundamental waves converged at the incident end facet 37a of the optical waveguide 37 is fed back to the semiconductor laser 32. In this case, the wavelength of the fundamental waves radiated from the semiconductor laser 32 varies about 1 nm after a short time. Therefore, the stable change period of the fundamental waves to the second harmonic waves is no more than several seconds. Accordingly, the stabilization of the wavelength of the fundamental waves is required to stably generate the blue light in the conventional shorter wavelength laser beam generating apparatus 31. FOURTH PREVIOUSLY PROPOSED ART: To stably change fundamental waves to second harmonic waves with a wavelength changing device according to the quasi-phase matching, a wavelength changing device having a plurality of gratings periodically arranged has been proposed (K. Shinozaki, et al., "Self-Quasi-Phase-Matched Second-Harmonic Generation in the Proton-Exchanged LiNbO 3 Optical Waveguide with Periodically Domain-Inverted Regions", Appl. Phys. Lett., Vol.59, No.29, p.510-512(1991)). Fig. 6 is a constitutional view of another conventional shorter wavelength laser beam generating apparatus in which a conventional wavelength changing device of Shinozaki is arranged. As shown in Fig. 6, a conventional shorter wavelength laser beam generating apparatus 41 consists of a semiconductor laser 42, a conventional wavelength changing device 43 for changing 1.3 µm wavelength fundamental waves radiated from the semiconductor laser 42 to 0.65 µm wavelength second harmonic waves, a spectrum analyzer 44 for analyzing the wavelength of the fundamental waves radiated from the semiconductor laser 42, and two pairs of optical lenses 45 for converging the fundamental waves radiated from the semiconductor laser 42 at single mode fibers connected to the wavelength changing device 43 and the spectrum analyzer 44. The wavelength changing device 43 consists of a polarized LiNbO 3 substrate 46, an optical waveguide 47 having inverted polarization layers 48 (or domain-inverted regions) periodically arranged at regular intervals Λ. Regions between the inverted polarization layers 48 are called non-inverted polarization layers 49 for convenience. In the optical waveguide 47, mismatching between a propagation constant of the fundamental waves and another propagation constant of the second harmonic waves is compensated by alternate rows of the inverted and non-inverted polarization layers 48, 49. This is, because the difference in the propagation constant between the fundamental waves and the second harmonic waves occurs, the phase of the fundamental waves agrees with that of the second harmonic waves in the optical waveguide 47 each time the fundamental waves transmit a minimum distance. Therefore, in cases where the regular intervals Λ of the inverted polarization layers 48 agree with a multiple of the minimum distance, the quasi-phase matching condition Λ = λ f /{2*(N2ω-Nω)} is satisfied, and the fundamental waves are changed to the second harmonic waves. The condition that the regular intervals Λ of the inverted polarization layers 48 agree with the minimum distance is called a first-order quasi-phase matching. Also, the condition that the regular intervals Λ agree with N times minimum distance is called an Nth-order quasi-phase matching. In the above configuration, fundamental waves having various wavelengths around 1.3 µm are radiated from the semiconductor laser 42 and are converged at the optical waveguide 47 through the optical lenses 45 and the single mode fiber. In the optical waveguide 47, quasi-phase matching (QPM) fundamental waves having a QPM wavelength satisfying the quasi-phase matching condition are selectively changed to second harmonic waves, and the second harmonic waves are efficiently amplified and output from the optical waveguide 47. Therefore, the QPM fundamental waves are selectively changed to the second harmonic waves in the wavelength changing device 43. In addition, because an effective refractive index of the inverted polarization layers 48 is slightly higher than another effective refractive index of the non-inverted polarization layers 49, a periodic structure in the effective refractive index consisting of the inverted polarization layers 48 and the non-inverted polarization layers 49 is produced in the optical waveguide 47. Therefore, a plurality of gratings are substantially formed in the optical waveguide 47. A group of the gratings substantially formed functions as a distributed Bragg reflector on condition that the DBR condition Λ = mλ/2N is satisfied. That is, DBR fundamental waves having a DBR wavelength satisfying the DBR condition are selectively reflected in the gratings. Thereafter, the reflected DBR fundamental waves are fed back to the semiconductor laser 42. Therefore, the wavelength of the fundamental waves radiated from the semiconductor laser 42 is fixed to the DBR wavelength. Accordingly, in cases where the DBR wavelength of the DBR fundamental waves reflected in the periodic structure functioning as the distributed Bragg reflector agrees with the QPM wavelength of the QPM fundamental waves, the change of the fundamental waves to the second harmonic waves can be stably performed in the conventional shorter wavelength laser beam generating apparatus 41. To achieve an agreement of the DBR wavelength of the reflected DBR fundamental waves and the QPM wavelength of the QPM fundamental waves, regular intervals Λ of the inverted polarization layers 48 periodically arranged are set to 13 µm µm. In this case, the wavelength of the fundamental waves radiated from the semiconductor laser 42 is fixed to 1.327 µm, and 1.327/2 µm wavelength second harmonic waves are stably generated. Also, the alternate rows of the inverted and non-inverted polarization layers 48, 49 becomes a first-order in the QPM structure, and the gratings functioning as the distributed Bragg reflector becomes a forty-third order in the DBR periodic structure. The grating order m is defined as an equation m= Λ/(λ f /2N). Here the symbol A denotes the regular intervals of the inverted polarization layers 48, the symbol λ f denotes a wavelength of the fundamental waves, and the symbol N denotes an effective averaged refractive index of the optical waveguide 47 for the fundamental waves. In cases where the pumping power of the fundamental waves converged at the optical waveguide 47 is 60 µW and the length of the optical waveguide 47 is 2 mm, the output power of the second harmonic waves is 0.652 pW. However, because the inverted polarization layers 48 periodically arranged function as a distributed Bragg reflector grating in the conventional shorter wavelength laser beam generating apparatus 41, a propagation speed of the fundamental waves and another propagation speed of the second harmonic waves are required to be controlled with high accuracy to achieve the agreement of the DBR wavelength of the DBR fundamental waves and the QPM wavelength of the QPM fundamental waves. Also, the range of the wavelength of the fundamental waves changed in the apparatus 41 is limited. Therefore, even though 1. 3 µm wavelength fundamental waves can be stably changed to 0. 65 µm wavelength second harmonic waves, there is a drawback that shorter wavelength second harmonic waves (the wavelengths range from 400 nm to 500 nm) useful in various fields are difficult to be generated in the apparatus 41. Also, because the inverted polarization layers 48 periodically arranged are utilized as the distributed Bragg reflector in the conventional shorter wavelength laser beam generating apparatus 41, the grating order in the DBR periodic structure becomes large in the apparatus 41. For example, in cases where the alternate rows of the inverted and non-inverted polarization layers 48, 49 is equivalent to the first-order in the QPM structure, the periodic structure functioning as the distributed Bragg reflector is equivalent to a several tens of grating order in the DBR periodic structure. Therefore, the fundamental waves are coupled to various radiation modes in the optical waveguide 47. The radiation modes consists of N types of radiation modes from a first radiation mode corresponding to the first grating order to an Nth radiation mode corresponding to an Nth grating order in cases where the periodic structure of the inverted polarization layers 48 is equivalent to the Nth grating order. Thereafter, the fundamental waves are radiated to various directions without being changed to the second harmonic waves while being led by the various radiation modes. As a result, the fundamental waves attenuates in the optical waveguide 47, and a radiating loss of the fundamental waves is increased. Accordingly, because the fundamental waves contributing the generation of the second harmonic waves are decreased by the increase of the radiating loss, there is a drawback that a changing efficiency of the fundamental waves to the second harmonic waves deteriorates. This drawback is illustrated in Fig. 7. Fig. 7 graphically shows a relationship between a reflection efficiency of the fundamental waves and the grating order and another relationship between a radiation loss of the fundamental waves and the grating order. As shown in Fig. 7, in cases where the gratings are arranged in a tenth grating order periodic structure, the reflection efficiency is only 10 %, and the radiation loss is no less than 75 %. Therefore, in cases where the grating order of periodic structure in the the distributed Bragg reflector grating is equal to or more than third grating order, the radiation loss of the fundamental waves is too many so that the conventional shorter wavelength laser beam generating apparatus 41 is not useful in practical use. In addition, higher grating order of the DBR periodic structure adversely influences on not only the fundamental waves but also the second harmonic waves generated in the optical waveguide 47 to increase a radiation loss of the second harmonic waves. Therefore, the second harmonic waves are scattered and reflected in the optical waveguide 47 to decrease the second harmonic waves radiated from an output end facet 47b of the optical waveguide 47. As a result, there is a drawback that the changing efficiency of the fundamental waves to the second harmonic waves moreover deteriorates. Accordingly, a wavelength changing device having the DBR periodic structure of a lower grating order (a first grating order or a second grating order) is required to change the fundamental waves to the second harmonic waves at high efficiency in practical use. A first aim of the present invention is to provide, with due consideration to the drawbacks of such a conventional diffracting device, a diffracting device in which coherent light is reflected at high reflection efficiency while reducing a transmission loss of the fundamental waves, and a method in which the diffracting device is manufactured according to simple processes at high reprobability. A second aim of the present invention is to provide a wavelength changing device in which a plurality of gratings periodically arranged are uniformely formed on a substrate having an optical waveguide to efficiently change fundamental waves to second harmonic waves and to stably generate the second harmonic waves, and a manufacturing method of the wavelength changing device in which a plurality of gratings are uniformly arranged on a substrate having an optical waveguide to efficiently reflect fundamental waves and to efficiently change the fundamental waves to second harmonic waves. A third aim of the present invention is to provide a laser beam generating apparatus in which a laser beam having a fixed wavelength is stably generated with the diffracting device or the wavelength changing device. A fourth aim of the present invention is to provide an optical information processing apparatus in which information is optically read or written with the laser beam generating apparatus. According to the present invention there is provided a diffracting device, comprising: a substrate; an optical waveguide arranged in the substrate for transmitting coherent light in a propagation direction a first cross-sectional area of the optical waveguide positioned at a reflecting region being smaller than a second cross-sectional area of the optical waveguide positioned at a non-reflecting region; a plurality of resist elements having a first refractive index N1 and periodically arranged adjacent to the optical waveguide of the reflecting region at regular intervals in the propagation direction of the coherent light transmitting through the optical waveguide, the resist elements being made of a soft material which has high workability; and a reflecting element having a second refractive index N2 higher than the first refractive index N1 of the resist elements and arranged between the resist elements for reflecting the coherent light distributed in the reflecting element, a refractive change being made by a periodic structure consisting of the resist element and the reflecting element, and overlap between the coherent light and the reflecting element being, in use, increased in the reflecting region. In the above configuration, coherent light radiated to the incident side of the optical guide transmits through the optical waveguide towards the output side thereof. In this case, a part of the coherent light is distributed in the periodic structure consisting of the covering layer and the gratings. Also, the refractive index of the gratings differs from that of the covering layer. Therefore, the periodic structure functions as a diffraction grating, and the coherent light is reflected in the periodic structure. As a result, in cases where the regular intervals of the gratings are equal to a multiple of a half wavelength of the coherent light, the reflected light is amplified and output from the incident side of the optical guide. A reflection efficiency is increased as the regular intervals of the gratings become shorter because the number of the gratings is increased. Therefore, a minute periodic structure is required of the diffracting device to enhance the reflection efficiency. Where a wavelength λ of the coherent light, an averaged refractive index N of the periodic structure, and the grating order m of the diffracting device are defined, the regular intervals Λ of the gratings satisfy an equation Λ = m*λ/(2N). Specifically, when the wavelength λ=800 nm and the averaged refractive index N=2 are given, the regular intervals Λ in a first order grating (m=1) is 0.2 µm. The value 0.2 µm is very small. Also, the reflection efficiency is increased as the height of the gratings is large because a reflecting area in each of the gratings is increased. In the present invention, because the soft material is utilized as a material of the gratings, the regular intervals of the gratings can be easily shortened. For example, in cases where a photoresist material is utilized as a material of the gratings, the gratings can be minutely patterned according to an interference-exposure process. Therefore, the periodic structure of a low grating order such as a first grating order or a second grating order can be easily manufactured with high accuracy. Also, because any etching process in which the soft material is etched by reactive ions is not required to minutely pattern the soft material, the optical waveguide is not damaged by the reactive ions. In addition, the gratings can be deeply formed because the soft material has high workability. Accordingly, the diffracting device according to the present invention has superior reflection efficiency. Also, a transmission loss is lowered in the diffracting device according to the present invention. In addition, because the gratings are covered by the covering layer, the covering layer functions as a protector for protecting the gratings from the atmosphere. Therefore, the superior reflection efficiency of the diffracting device can be maintained for a long time. The invention further provides a method for manufacturing a diffracting device, comprising the steps of: forming an optical waveguide in a substrate such that a first cross-sectional area of the optical waveguide positioned at a reflecting region is smaller than a second cross-sectional area of the optical waveguide positioned at a non-reflecting region, coherent light, in use, transmitting through the first and second cross-sectional areas of the optical waveguide in a propagation direction; patterning a resist soft material having a high workability placed on the first cross-sectional area of the optical waveguide to form a plurality of resist elements having a first refractive index N1 periodically arranged at regular intervals in the propagation direction of the coherent light, and arranging a reflecting element having a second refractive index N2 higher than the first refractive index N1 of the resist elements between the resist elements to form a refractive change in a periodic structure consisting of the resist elements and the reflecting element and to reflect the coherent light distributed in the reflecting element in each of alternate rows of the resist elements and the reflecting element overlap between the coherent light and the reflecting element, in use, being increased in the reflecting region. In the above steps, as the regular intervals of the gratings becomes shorter, the number of the gratings is increased. Therefore, coherent light transmitting the optical waveguide is reflected more times in the alternate rows of the covering layer and the gratings, and a reflection efficiency is increased. Therefore, a minute periodic structure is required to enhance the reflection efficiency in the diffracting device. Also, the reflection efficiency is increased as the height of the gratings is large because a reflecting area in each of the gratings is increased. Therefore, the gratings deeply formed are required to enhance the reflection efficiency in the diffracting device. In the present invention, because the soft material is utilized as a material of the gratings, the regular intervals of the gratings can be easily shortened. For example, in cases where a photoresist material is utilized as a material of the gratings, the gratings can be minutely patterned according to an interference-exposure process. That is, the photoresist film arranged on the optical waveguide is exposed to interference-exposing light, and grating patterned areas of the photoresist film becomes soluble in a developer solution. Thereafter, the photoresist film is developed in the developer solution to remove the grating patterned areas of the photoresist film exposed. Therefore, the gratings are formed on the optical waveguide. In this case, because the photoresist film is exposed to interference-exposing light, regular intervals of the gratings can be easily shortened to about 0.2 µm. Accordingly, a large number of gratings can be arranged in a limited area to enhance the reflection efficiency. Also, in cases where the soft material is deeply arranged on the optical waveguide, the gratings deeply formed can be easily arranged because the soft material has high workability. In addition, because any etching process in which the soft material is etched by reactive ions is not required to minutely pattern the soft material, the optical waveguide is not damaged by any reactive ion. Therefore, the surface of the optical waveguide is smooth. Also, because two superimposed masks are not utilized, the gratings can be reliably uniform in size, and the regular intervals of the gratings can be reliably the same as one another. Accordingly, the coherent light transmitting through the optical waveguide can be selectively reflected by the alternate rows of the covering layer and the gratings with high accuracy. In addition, because the gratings are covered by the covering layer, the covering layer functions as a protector for protecting the gratings from the atmosphere. Therefore, the superior reflection efficiency of the diffracting device can be maintained for a long time. The present invention also provides a wavelength changing device incorporating the diffracting device described above. In this device, a part of fundamental waves radiated to the optical waveguide change to second harmonic waves, of which a wavelength λ h is half of another wavelength λ f of the fundamental waves, in the alternate rows of the inverted polarization layers and the non-inverted polarization layers. Thereafter, the second harmonic waves are output from an output end facet of the optical waveguide. Also, the fundamental waves not changing to the second harmonic waves are reflected by the periodic structure of the covering layer and the gratings because the first refractive index N1 of the gratings differs from the second refractive index of the covering layer to form a refractive change functioning as a diffraction grating. The fundamental waves reflected are output from an incident end facet of the optical waveguide and are fed back to a fundamental wave source such as a semiconductor laser. Therefore, the wavelength λ f of the fundamental waves radiated from the fundamental source is fixed. Accordingly, the fundamental waves stably change to the second harmonic waves. A reflection efficiency is increased as the grating intervals of the gratings become shorter because the number of the gratings is increased. Therefore, a minute periodic structure is required of the wavelength changing device to enhance the reflection efficiency. Where an averaged refractive index N of the periodic structure and the grating order m of the periodic structure are defined, the regular intervals Λ1 of the gratings satisfy an equation Λ1 = m*λ f /(2N). Specifically, when the wavelength λ f =800 nm and the averaged refractive index N=2 are given, the regular intervals Λ1 in a first order grating (m=1) is 0.2 µm. The value 0.2 µm is very small. Also, the reflection efficiency is increased as the height of the gratings is large because a reflecting area in each of the gratings is increased. In the present invention, because the soft material is utilized as a material of the gratings, the grating intervals of the gratings can be easily shortened. For example, in cases where a photoresist material is utilized as a material of the gratings, the gratings can be minutely patterned according to an interference-exposure process. Therefore, the periodic structure of a low grating order such as a first grating order or a second grating order can be easily manufactured with high accuracy. Also, because any etching process in which the soft material is etched by reactive ions is not required to minutely pattern the soft material, the optical waveguide is not damaged by the reactive ions. In addition, the gratings can be deeply formed because the soft material has high workability. Accordingly, the wavelength changing device according to the present invention can have superior reflection efficiency. Also, a transmission loss is lowered. In addition, because the gratings are covered by the covering layer, the covering layer functions as a protector for protecting the gratings from the atmosphere. Therefore, the superior reflection efficiency can be maintained for a long time. The present invention also provides a laser beam generating apparatus, including the diffracting device described above. When a driving current supplied to a semiconductor laser or an ambient temperature varies, a refractive index of material of the semiconductor laser also varies. As a result, the wavelength of coherent light radiated from the semiconductor laser generally changes. For example, a first driving current supplied to the semiconductor laser to read information stored in an optical disk greatly differs from a second driving current supplied to the semiconductor laser to write information in the optical disk. Therefore, a focal point of an objective lens utilized to converge the coherent light at the optical disk conventionally changes each time a reading operation and a writing operation are exchanged to each other. To avoid adverse influence of the change in the focal point, the adjustment of the focal point is conventionally required. In the linear beam generating apparatus of the invention, because a part of coherent light radiated from the semiconductor laser is fed back to the semiconductor laser by the function of the diffracting device. the wavelength of the coherent light radiated from the semiconductor laser is fixed. Therefore, even though the driving current or the ambient temperature varies, the focal point does not change. Therefore, the exchange between the reading operation and the writing operation can be quickly performed without any adjustment of the focal point. Accordingly, lens material having a large wavelength dispersion coefficient can be utilized in the laser beam generating apparatus. Also, a lens having a large numerical aperture (NA) can be utilized. The present invention also provides an optical information processing apparatus including a diffracting device as described above. In the optical information processing apparatus of the invention, the wavelength of the coherent light radiated from the semiconductor laser is fixed because the coherent light is fed back to the semiconductor laser with the diffracting device. Thereafter, the coherent light having a fixed wavelength is converged at the information medium by the optical system, and the intensity of the coherent light is detected to read the information stored in the information medium. Accordingly, because the wavelength of the coherent light is fixed even though an ambient temperature varies, a focal length of the optical system does not vary. Therefore, the information can be reliably read. Also, even though a driving current supplied to the semiconductor laser varies, the wavelength of the coherent light is stably fixed. Therefore, even though a reading operation and a writing operation are exchanged to each other, the adjustment of the focal length is not required. The present invention will be further explained in the following description of exemplary embodiments and the accompanying drawings, in which: Fig. 1 is a diagonal view of a conventional diffracting device having a distributed Bragg reflector; Figs 2A to 2D are cross-sectional views of the diffracting device shown in Fig. 1, showing a manufacturing method of the diffracting device; Fig. 3 graphically shows transmitting and reflecting characteristics of the diffracting device shown in Fig. 1; Fig. 4 is a cross-sectional view of another conventional diffracting device; Fig. 5 is a constitutional view of a conventional shorter wavelength laser beam generating apparatus; Fig. 6 is a constitutional view of another conventional shorter wavelength laser beam generating apparatus in which a conventional wavelength changing device of Shinozaki is arranged; Fig. 7 graphically shows a relationship between a reflection efficiency of fundamental waves and the grating order substantially formed in the wavelength changing device shown in Fig. 6 and another relationship between a radiating loss of the fundamental waves and the grating order; Fig. 8A is a diagonal perspective view of a diffracting device according to a first embodiment of the present invention; Fig. 8B is a cross-sectional view of the diffracting device shown in Fig. 8A to illustrate intensity distribution of coherent light transmitting through an optical waveguide. Figs. 9A to 9D are diagonal views showing a manufacturing method of the diffracting device shown in Fig. 8A; Fig. 10 is a constitutional view of an optical system for measuring optical characteristics of the diffracting device shown in Fig. 8A; Fig. 11 shows a relationship between the intensity of reflected coherent light and the wavelength of the coherent light and another relationship between the intensity of transmitting coherent light and the wavelength of the coherent light, those relationships being measured with the optical system shown in Fig. 10; Fig. 12 shows the comparison in optical characteristics between the diffracting device shown in Fig. 8 and samples 1, 2; Fig. 13A is a diagonal perspective view of a diffracting device according to a second embodiment of the present invention; Fig. 13B is a cross-sectional view of the diffracting device shown in Fig. 13A to illustrate intensity distribution of coherent light transmitting through an optical waveguide; Fig. 14A is a diagonal perspective view of a diffracting device according to a modification of the second embodiment; Fig. 14B is a cross-sectional view of the diffracting device shown in Fig. 14A to illustrate intensity distribution of coherent light transmitting through an optical waveguide; Fig. 15 is a diagonal perspective view of a wavelength changing device according to a third embodiment of the present invention; Fig. 16A is a diagonal perspective view of a wavelength changing device according to a fourth embodiment of the present invention; Fig. 16B is a cross-sectional view of the wavelength changing device shown in Fig. 16A; Fig. 17 is a constitutional view of an optical system for examining optical characteristics of the wavelength changing device shown in Fig. 16A; Fig. 18, 19 graphically show the relation between the intensity of second harmonic waves P2 and a temperature of a semiconductor laser shown in Fig. 17, depending on the ambient temperature; Fig. 20A is a diagonal perspective view of a wavelength changing device according to a fifth embodiment of the present invention; Fig. 20B is a cross-sectional view of the wavelength changing device shown in Fig. 20A; Fig. 21 shows influence of the positional relation between a wave reflecting region and a wavelength changing region on optical characteristics of a wavelength changing device shown in Fig. 20B; Fig. 22 is a cross-sectional view of a wavelength changing device according to a sixth embodiment; Fig. 23 graphically shows relation between a ratio W1/Λ1 and a transmission loss for fundamental waves and relation between a ratio W1/Λ1 and a reflection efficiency for fundamental waves; Fig. 24 is a constitutional view of a shorter wavelength laser beam generating apparatus according to a seventh embodiment of the present invention; Fig. 25 is a diagonal view of a shorter wavelength laser beam generating apparatus according to a eighth embodiment of the present invention; Fig. 26 graphically shows relation between wavelength of fundamental waves and electric potential applied to a first electrode shown in Fig. 25; Fig. 27 is a cross-sectional view of a laser beam generating apparatus according to a ninth embodiment of the present invention; Figs. 28A to 28C are cross-sectional views showing a manufacturing method of the diffracting device shown in Fig. 27; Fig. 29 graphically shows relation between driving current supplied to a semiconductor laser and wavelength λ c of coherent light radiated from the semiconductor laser shown in Fig. 27; Fig. 30 is a constitutional view of a laser beam generating apparatus according to a tenth embodiment of the present invention; Fig. 30A is a diagonal perspective view of a diffracting device shown in Fig. 30; Fig. 31B is a cross-sectional view of the diffracting device shown in Fig. 31A; Fig. 32 is a constitutional view of an optical information processing apparatus according to a eleventh embodiment of the present invention; Fig. 33 is a constitutional view of a shorter wavelength laser beam generating apparatus according to a twelfth embodiment of the present invention; Fig. 34 is a constitutional view of an optical information processing apparatus according to the twelfth embodiment of the present invention; Fig. 35 is a constitutional view of a laser beam generating apparatus according to a thirteenth embodiment of the present invention. DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a diffracting device according to the present invention are described with reference to drawings. 1. FIRST EMBODIMENT: A first embodiment is described with reference to Figs. 8 to 12. Fig. 8A is a diagonal perspective view of a diffracting device according to a first embodiment of the present invention. Fig. 8B is a cross-sectional view of the diffracting device shown in Fig. 8A to illustrate intensity distribution of coherent light transmitting through an optical waveguide. As shown in Figs. 8A, 8B, a diffracting device 51 comprises a LiTaO 3 substrate 52, an optical waveguide 53 extending in a central upper side of the LiTaO 3 substrate 52 for transmitting coherent light consisting of 860 nm wavelength fundamental waves P1 from an incident end facet 53a to an output end facet 53b, a series of gratings 54 periodically arranged on the LiTaO 3 substrate 52 at first regular intervals Λ1 for reflecting the coherent light transmitting through the optical waveguide 53, and a covering layer 55 covering the gratings 54 for protecting the gratings 54. The LiTaO 3 substrate 52 is formed by cutting out LiTaO 3 crystal in a direction perpendicular to a Z-axis defined as [001]-direction in Miller indices. Therefore, the LiTaO 3 substrate 52 (or -Z plate) has an upper surface defined as (001)-plane in Miller indices. The optical waveguide 53 is formed by exchanging a part of Li + ions of the LiTaO 3 substrate 52 for H + ions. Therefore, an effective refractive index of the optical waveguide 53 is slightly higher than that of the LiTaO 3 substrate 52 to confine a large part of the coherent light in the optical waveguide 53. A width of the optical guide 53 is 4 µm, and a depth of of the optical guide 53 is 2 µm. The gratings 54 are made of a photoresist material (manufactured by Shiply Ltd., and product No. AZ1400-17). Because the photoresist material is a radiation-sensitive compound and a soft material having high workability, the grating 54 can be minutely formed by exposing the photoresist material to exposing light and developing the photoresist material to remove exposed areas of the photoresist material. That is, the photoresist can be minutely patterned to form the gratings 54 without being etched by reactive ions. An effective refractive index N1 of the gratings 54 is equal to 1.5. Also, the gratings 54 have the same size as one another, and the gratings 54 cross over the optical waveguide 53 to reflect the coherent light distributed over the optical waveguide 53. The covering layer 55 is made of Ta 2 O 5 of which an effective refractive index N2 is equal to 2.0. Because the covering layer 55 is arranged between the gratings 54, the change of the effective refractive index is generated in a periodic structure consisting of the gratings 54 and the covering layer 55. Therefore, the periodic structure is equivalent to a diffraction grating, and the periodic structure functions as the distributed Bragg reflector on condition that the DBR condition is satisfied. Also, because a refractive difference in the effective refractive index between the gratings 54 and the covering layer 55 is large, a reflection efficiency of the periodic structure for the coherent light becomes large. Therefore, the combination of the gratings 54 made of the photoresist and the covering layer 55 made of Ta 2 O 5 effectively functions as the diffraction grating. As shown in Fig. 8B, the coherent light transmitting through the optical waveguide 53 is also distributed in the periodic structure and the substrate 52. The reflection efficiency of the periodic structure is generally increased in proportional to the height of the gratings 54 and the refractive difference in the effective refractive index. Also, because the grating order of the gratings 55 is proportional to regular intervals Λ1 of the gratings 55, the reflection efficiency is inversely proportional to the regular intervals Λ1 of the gratings 55. Next, a manufacturing method of the diffracting device 51 is described. Figs. 9A to 9D are diagonal views showing a manufacturing method of the diffracting device 51 shown in Fig. 8. As shown in Fig. 9A, a Ta film 56 is deposited on the LiTaO 3 substrate 52 with a sputtering method, and a stripe hole 56a is formed on a central surface of the LiTaO 3 substrate 52 according to a photolithography process. The width of the stripe hole 56a is 4 µm, and the length of the stripe hole 56a is 5 mm. Thereafter, the LiTaO 3 substrate 52 is immersed in a pyrophosphoric acid (H 4 P 2 O 7 ) solution for fourteen minutes at a temperature of 260 °C to exchange a part of Li + ions of the LiTaO 3 substrate 52 not deposited by the Ta film 56 for H + ions, according to a proton-exchange process. Thereafter, the LiTaO 3 substrate 52 is annealed for sixty seconds at a temperature of 420 °C to form the optical waveguide 53 having a superior transmission efficiency. Thereafter, the Ta film 56 is taken away. The width of the optical waveguide 53 is 4µm, and the depth is 2 µm. A guided wave loss of the optical waveguide 53 for the coherent light is only 1.0 dB/cm. Thereafter, both end facets of the optical waveguide 53 are optically polished to form the incident end facet 53a and the output end facet 53b. Thereafter, as shown in Fig. 9B, diluted photoresist 57 (AZ1400-17) is coated over the LiTaO 3 substrate 52 and the optical waveguide 53. The thickness of the photoresist 57 coated is 0.2 µm. Thereafter, grating pattern areas of the photoresist 57 are exposed to 0.4416 nm wavelength light radiated from He-Cd laser according to an interference-exposure process to transfer a grating pattern to the photoresist 57. Therefore, the photoresist 57 exposed becomes soluble in a developer solution. Thereafter, the photoresist 57 is immersed in the developer solution to develop the photoresist 57. Therefore, the grating patterned areas of the photoresist 57 exposed are removed. Therefore, photoresist portions formed in the grating pattern are arranged on the optical waveguide 53. Thereafter, the photoresist portions are cured so that the gratings 54 crossing over the optical waveguide 53 are formed, as shown in Fig. 9C. The regular intervals Λ1 of the gratings 54 periodically arranged are set to 0.4 µm, a grating height is set to 0.2 µm, a ratio of the grating width W1 to the regular intervals Λ1 is set to 0.23. and a total length of the gratings 54 in a propagation direction of the coherent light is set to 5 mm. Thereafter, as shown in Fig. 9D, Ta 2 O 5 is deposited over the gratings 54 with a sputtering method to form the covering layer 55. The height of the covering layer 55 deposited on the gratings 54 is 0.3 µm in thickness. Therefore, the covering layer 55 protects the gratings 54 from the atmosphere. Next, characteristic estimation results of the diffracting device 51 are described. To estimate optical characteristics of the diffracting device 51, an optical system 61 is prepared as shown in Fig. 10. The optical system 61 comprises a Ti-Al 2 O 3 laser 62 for radiating coherent light of which the wavelength is variable, a half mirror 63 for dividing the coherent light, an optically converging system 64 for converging the coherent light at the incident end facet 53a of the diffracting device 51, the diffracting device 51, a collimator lens 65 for collimating the coherent light which transmits through the optical waveguide 53 without being reflected by the gratings 54, a transmitting light detector 66 for detecting the intensity of the coherent light transmitting through the optical waveguide 53, and a reflected light detector 67 for detecting the intensity of the coherent light reflected by the gratings 54. In the above configuration, coherent light radiated from the Ti-Al 2 O 3 laser 62 transmits through the half mirror 63 and is converged at the incident end facet 53a of the optical wave guide 53 by the optically converging system 64. Thereafter, the coherent light transmits through the optical waveguide 53. In this case, because a part of coherent light is distributed in the covering layer 55 as shown in Fig. 8B, the part of coherent light is reflected by the covering layer 55. Therefore, the coherent light reflected is returned to the incident end facet 53a of the optical waveguide 53 and transmits through the optically converging system 64. Thereafter, the reflected coherent light is divided by the half mirror 63 and is detected by the reflected light detector 67. In contrast, the coherent light not reflected by the covering layer 55 is radiated from the output end facet 53b of the optical waveguide 53 and is collimated by the collimator lens 66. Thereafter, the coherent light not reflected (called transmitting coherent light) is detected by the transmitting light detector 66. When the wavelength of the coherent light radiated from the Ti-Al 2 O 3 laser 62 agrees with a distributed Bragg reflector wavelength (called DBR wavelength hereinafter), the intensity of the reflected cohelent light is remarkably increased. The DBR wavelength λ is determined by the regular intervals Λ1 of the gratings 54 and an effective refractive index N (=2.15) of the optical waveguide 53. That is, a DBR condition is designated by an equation λ = (Λ1*2N)/m. Here the symbol m (=2) denotes a grating number. Fig. 11 shows a relationship between the intensity of the reflected coherent light and the wavelength of the coherent light and another relationship between the intensity of the transmitting coherent light and the wavelength of the coherent light. As shown in Fig. 11, the DBR wavelength is 860 nm, and a diffraction efficiency (or a reflection efficiency) for the coherent light is about 50 %. Here the diffraction efficiency is defined as an intensity ratio of the reflected coherent light detected by the reflected light detector 67 to the coherent light coupled to the optical waveguide 53. Because a theoretical value of the diffraction efficiency is 60 %, the experimental value 50 % of the diffraction efficiency is high and superior to that obtained in a conventional diffracting device. The reason why the experimental value is superior is as follows. The gratings 53 is formed by developing the photoresist 57 without being etched so that the optical waveguide 53 is not damaged by any reactive ions. Also, because the gratings 54 having a superior uniformity is formed by exposing the photoresist 57 to the 0.4416 µm wavelength light according to the interference-exposure process, the gratings 54 can be formed in superior uniformity in size. Also, a full width at half maximum (FWHM) indicating the dependence of the reflected coherent light on the wavelength is only 0.03 nm. Because the full width at half maximum of the reflected coherent light also indicates the uniformity in the shape of the gratings 54, the small value of the full width at half maximum proves that the gratings 54 is formed in superior uniformity. Next, a comparison in optical characteristics between diffracting devices manufactured by conventional methods and the diffracting device 51 is described. A sample 1 equivalent to the conventional diffracting device 11 shown in Fig. 1 is prepared, and a sample 2 equivalent to the conventional diffracting device 21 shown in Fig. 4 is prepared. A manufacturing method of the sample 1 is as follows. After the Ta film 56 shown in Fig. 9A is taken away, the optical waveguide 53 is directly etched by reactive ions according to a dry etching method, and a plurality of gratings are formed in the optical waveguide 53 to manufacture a first conventional diffracting device (sample 1). Regular intervals of the gratings are 0.4 µm, and the depth is 0.1 µm. A manufacturing method of the sample 2 is as follows. After the Ta film 56 shown in Fig. 9A is taken away, Ta 2 O 5 is deposited on the optical waveguide 53 and the LiTaO 3 substrate 52 with a sputtering method. The thickness of a Ta 2 O 5 film deposited is 0.1 µm. Thereafter, a resist is coated on the Ta 2 O 5 film with a thickness of 0.2 µm, and the resist is exposed to 0.4416 µm wavelength light radiated from a He-Cd laser according to an interference-exposure process to form a grating pattern in the resist. Thereafter, the resist exposed is developed and formed in the grating pattern. Thereafter, the Ta 2 O 5 film is etched by reactive ions through the patterned resist functioning as a mask in a dry-etching apparatus, so that a plurality of gratings formed of the Ta 2 O 5 film etched are arranged on the optical waveguide 53. Regular intervals of the gratings are 0.4 µm, and the depth is 0.1 µm. Fig. 12 shows the comparison in optical characteristics between the diffracting device 51 and the samples 1, 2. To manufacture the sample 1, complicated processes which are performed with two superimposed masks shown in Fig. 2A to 2D are required. Therefore, the uniformity in the shape of the gratings is inferior. Also, because the reactive ions are injected into the optical waveguide to etch the optical waveguide, the surface of the optical waveguide becomes rough. Therefore, a guided wave loss of the coherent light transmitting through the optical waveguide is increased. As a result, a reflection efficiency (or a diffraction efficiency) of the conventional diffracting device (sample 1) is decreased to 5 %, and a transmission efficiency of the conventional diffracting device (sample 1) is are decreased to 10 %. Accordingly, the sample 1 is not useful for practical use. Also, to manufacture the sample 2, the Ta 2 O 5 film is etched by the reactive ions. Therefore, the surface of the optical waveguide is damaged by the reactive ions. As a result, a guided wave loss of the coherent light transmitting through the optical waveguide is increased, and the optical characteristics of the conventional diffracting device (sample 2) deteriorates. In contrast, because the photoresist 57 is not etched by any reactive ion in the diffracting device 51, the optical waveguide 53 is not damaged by any reactive ion. Therefore, a scattering loss of the coherent light is lowered so that a guided wave loss of the coherent light is lowered to 1 dB/cm. Also, because the covering layer 55 is deposited to cover the gratings 54 to protect the gratings 54 from the atmosphere, the gratings 54 are not degraded. Therefore, a reflection efficiency of the diffracting device 51 can be stably maintained with high degree. In addition, any complicated processes performed with two superimposed masks are not required to manufacture the diffracting device 51, the gratings 54 can be reliably formed in the same size, and the regular intervals Λ1 of the gratings 54 are the same. Also, in cases where the coating of the diluted photoresist 57 is thickened, the gratings 54 can be deeply formed by sufficiently exposing the photoresist 57 to the 0.4416 µm wavelength light. Therefore, a theoretical diffraction efficiency is increased, and an actual diffraction efficiency can be enhanced. Accordingly, because the gratings 54 made of the photoresist 57 are deeply formed with high accuracy without damaging the optical waveguide 53 in the diffracting device 51, the lowered guided wave loss and the high reflection efficiency for the coherent light transmitting through the optical waveguide 53 can be obtained in the diffracting device 51. Also, because the refractive index of the grating 54 is sufficiently small as compared with that of the covering layer 55, the grating order can be decreased. In other words, the number of gratings 54 arranged in a regular length can be increased by decreasing the regular intervals Λ1 of the gratings 54. Therefore, the reflection efficiency can be increased. In the first embodiment, the photoresist 57 made of AZ1400-17 manufactured by Shiply Ltd. is utilized as the material of the gratings 54. However, the material of the gratings 54 is not limited to AZ1400-17. That is, any photoresist is applicable on condition that the refractive index of the photoresist is sufficiently small as compared with that of the covering layer 55. Also, a burning type metallic oxide film is appliable in place of the photoresist 57. Specifically, a burned SiO 2 film obtained by burning SiO 2 is useful because an etching rate of the burned SiO 2 film is large. That is, a deep shape of gratings can be easily formed by etching the burned SiO 2 film through a resist functioning as a mask. Also, the position of the gratings 54 is not limited on the optical waveguide 53. That is, the gratings 54 adjacent to the optical waveguide 53 is useful as the distributed Bragg reflector. For example, the gratings 54 positioned under the optical waveguide 53 or the gratings 54 positioned at a side of the optical waveguide 53 is useful. Also, the covering layer 55 is made of Ta 2 O 5 in the first embodiment because the effective refractive index of Ta 2 O 5 is so large and because a transmitting loss of the coherent lightis low in Ta 2 O 5 . However, the material of the covering layer 55 is not limited to Ta 2 O 5 . That is, a material having an effective refractive index differing from that of the gratings 54 is applicable on condition that a transmitting loss for the coherent light is low. For example, TiO 2 and SiN can be applied in place of Ta 2 O 5 . Specifically, in cases where a material of which the effective refractive index is over 1.8 is utilized as the covering layer 55, the diffraction efficiency of the diffracting device 51 can be enhanced. Also, the LiTaO 3 substrate 52 has the upper surface indicated by the (001)-plane. However, a LiTaO 3 substrate having an upper surface indicated by a (100)-plane or a (010) plane can be applied. 2. SECOND EMBODIMENT: A second embodiment is described with reference to Figs. 13A, 13B. Fig. 13A is a diagonal perspective view of a diffracting device according to a second embodiment of the present invention. Fig. 13B is a cross-sectional view of the diffracting device shown in Fig. 13A to illustrate intensity distribution of coherent light transmitting through an optical waveguide. As shown in Fig. 13A, a diffracting device 71 comprises the LiTaO 3 substrate 52, an optical waveguide 72 extending in a central upper side of the LiTaO 3 substrate 52 for transmitting coherent light consisting of 860 nm wavelength fundamental waves P1 from an incident end facet 72a to an output end facet 72b, the gratings 54 periodically arranged on the LiTaO 3 substrate 52 for reflecting the coherent light transmitting through the optical waveguide 72, and the covering layer 55. The optical waveguide 72 is formed according to the proton-exchange process in the same manner as in the optical waveguide 53. Also, as shown in Fig. 13B, a depth D1 of the optical waveguide 72 positioned in the neighborhood of the incident end facet 72a (a non-reflecting region) is larger than a depth D2 of the optical waveguide 72 positioned in the neighborhood of the output end facet 72b (a reflecting region) on which the gratings 54 are periodically arranged at the regular intervals Λ1. Therefore, the intensity distribution of the coherent light shifts towards the periodic structure consisting of the gratings 54 and the covering layer 55 at the reflecting region (the depth D2) of the optical waveguide 72. as compared with that at the non-reflecting region (the depth D1) of the optical waveguide 72. The reflection efficiency of the periodic structure is generally proportional to the overlapping degree of the distributed coherent light and the periodic structure. In the diffracting device 71, because the depth D2 of the optical waveguide 72 is narrowed at the reflecting region, the overlapping degree is increased at the reflecting region. Therefore, the reflection efficiency for the coherent light is enhanced. The optical characteristics of the diffracting device 71 are described with reference to Fig. 12. The depth D2 of the optical waveguide 72 is 2 µm, the width of the optical waveguide 72 is 4 µm, the regular intervals Λ1 of the gratings 54 is 0.4 µm, and the height (or the depth) of the gratings 54 is 0.2 µm, in the same manner as in the first embodiment. Also, the depth D1 of the optical waveguide 72 is 1.8 µm. In this case, the reflection efficiency is 60 %, the transmission efficiency is 20 %, the guided wave loss is 2db/cm, and the full width at half maximum (FWHM) is 0.05 nm. Therefore, the reflection efficiency is enhanced as compared with in the first embodiment. In contrast, the guided wave loss is slightly increased because the optical waveguide 72 is narrowed. In the second embodiment, the position of the gratings 54 is not limited on the optical waveguide 53. That is, the gratings 54 adjacent to the optical waveguide 72 is useful as the distributed Bragg reflector. In this case, a cross-sectional area of the optical waveguide 72 in the reflecting region is smaller than that in the non-reflecting region to increase the overlapping degree between the distributed coherent light and the periodic structure. Next, a modification of the second embodiment is described with reference to Fig. 14. Fig. 14A is a diagonal perspective view of a diffracting device according to a modification of the second embodiment. Fig. 14B is a cross-sectional view of the diffracting device shown in Fig. 14A to illustrate intensity distribution of coherent light transmitting through an optical waveguide. As shown in Figs. 14A, B, a diffracting device 73 comprises the LiTaO 3 substrate 52, an optical waveguide 74 extending in a central upper side of the LiTaO 3 substrate 52 for transmitting coherent light consisting of 860 nm wavelength fundamental waves P1 from an incident end facet 74a to an output end facet 74b, and the covering layer 55. A plurality of grooves are digged on the surface of the optical waveguide 74 to form a plurality of gratings 74c periodically arranged in a propagation direction of the coherent light. A depth D3 of the optical waveguide 74 positioned at a non-reflecting region adjacent to the incident end facet 74a is larger than a depth D4 of the optical waveguide 74 positioned at a reflecting region on which the gratings 74c are periodically arranged. Therefore, the intensity distribution of the coheren light is shifted towards the periodic structure consisting of the gratings 74c and the covering layer 55 at the reflecting region of the optical waveguide 74, as compared with that at the non-reflecting region of the optical waveguide 74. Next, a manufacturing method of the diffracting device 73 is described. The optical waveguide 74 is initially formed in the upper side of the substrate 52 in a step shape according to the proton-exchange process in the same manner as in the first embodiment. The depth D3 of the optical waveguide 74 is 2.4 µm, and the depth D4 of the optical waveguide 74 is 2.2 µm. Thereafter, a thin Ti film is deposited on the substrate 52 and the optical waveguide 74. Thereafter, a photoresist is spin coated on the Ti film. Thereafter, the photoresist is exposed to interference light according to an interference-exposure process, and the photoresist is developed to remove exposed areas of the photoresist. Therefore, a periodic grating pattern is transferred to the photoresist. Thereafter, the Ti film is periodically etched at regular intervals Λ1=0.4 µm by reactive ions generated in an atmosphere of CCl 2 F 2 gas according to a reactive ion etching to transfer the periodic grating pattern of the photoresist to the Ti film. Thereafter, the patterned film is used as a mask, and the optical waveguide 74 is etched by reactive ions generated in an atmosphere of CF 4 according to the reactive ion etching. Therefore, the gratings 74c are periodically formed. The height of the gratings 74c is 0.1 µm, and the regular intervals Λ1 of the gratings 74c is 0.4 µm. Thereafter, both end facets 74a, 74b of the optical waveguide 74 are polished. Finally, Ta 2 O 5 is deposited on the optical waveguide 74 and the substrate 52 at the thickness of 0.3 µm to form the covering layer 55. Because the refractive index of Ta 2 O 5 is 2.0 which is near to that of the optical waveguide 74, a roughed surface of the optical waveguide 74 is uniformed by the deposition of Ta 2 O 5 . Therefore, a scattering loss of the coherent light is extremely reduced. Also, even though the thickness of the optical waveguide 74 is thinned by the ion etching, the guided wave loss of the coherent light is not increased because the depth of the optical waveguide 74 is deeper than that of the optical waveguide 72. The optical characteristics of the diffracting device 73 are described with reference to Fig. 12. The reflection efficiency is increased to 70 %, the transmission efficiency is decreased to 5 %, and the guided wave loss is maintained at 2db/cm.is 0.05 nm, as compared with in the diffracting device 71. The position of the gratings 54 is not limited on the optical waveguide 72. That is, the gratings 54 positioned at peripheries of the optical waveguide 72 is useful as the distributed Bragg reflector. For example, the gratings 54 positioned under the optical waveguide 72 or the gratings 54 positioned at a side of the optical waveguide 72 is useful. 3. THIRD EMBODIMENT: A third embodiment is described with reference to Fig. 15. Fig. 15 is a diagonal perspective view of a wavelength changing device according to a third embodiment of the present invention. As shown in Fig. 15, a diffracting device 76 comprises the LiTaO 3 substrate 52, the optical waveguide 53, the gratings 54, and a covering layer 77 covering the gratings 54 for protecting the gratings 54. The covering layer 77 is made of burning type metallic oxide such as TiO 2 . In detail, metallic oxide such as TiO 2 is held in solution in a solvent. Thereafter, the metallic oxide held in solution is coated on the LiTaO 3 substrate 52 and the optical waveguide 53, and the metallic oxide coated is heated at a temperature of 200 to 500 °C. Therefore, the solvent is vaporized, and a burned metallic oxide film is formed as the covering layer 77. Because any vacuum system including a sputtering apparatus is not required to form the burned metallic oxide film as the covering layer 77, the diffracting device 76 can be easily manufactured. In cases where the gratings 54 is made of burned SiO 2 , the total length of the gratings 54 in the propagation direction is 1 mm, and the periodic structure is equivalent to a first grating order (m=1), the reflection efficiency is 25 %. 4. FOURTH EMBODIMENT: A fourth embodiment is described with reference to Figs. 16 to 19. Fig. 16A is a diagonal perspective view of a wavelength changing device according to a fourth embodiment of the present invention. Fig. 16B is a cross-sectional view of the wavelength changing device shown in Fig. 16A. As shown in Fig. 16A, a wavelength changing device 81 comprises a substrate 82 made of non-linear optical crystal LiTaO 3 which is dielectrically polarized in a lower direction, an optical waveguide 83 extending in a central upper side of the substrate 82 for transmitting coherent light consisting of 860 nm wavelength fundamental waves P1 from an incident end facet 83a to an output end facet 83b, a plurality of inverted-polarization layers 84 arranged in an upper side of the substrate 82 at second regular intervals Λ2 to cross the optical waveguide 83, the gratings 54 periodically arranged on the optical waveguide 83 at the first regular intervals Λ1, and the covering layer 55. The gratings 54 are positioned in the neighborhood of the output end facet 83b. The substrate 82 is formed by cutting out LiTaO 3 crystal in a direction perpendicular to a Z-axis defined as [001]-direction in Miller indices. Therefore, the LiTaO 3 substrate 82 (or -Z plate) has an upper surface defined as (001)-plane in Miller indices. The optical waveguide 83 is formed at a depth of 2 µm by exchanging a part of Li + ions of the substrate 82 for H + ions. Therefore, an effective refractive index of the optical waveguide 83 is slightly higher than that of the substrate 82 to confine large parts of the fundamental waves P1 in the optical waveguide 83. A width of the optical waveguide 83 is 4 µm. The inverted-polarization layers 84 dielectrically polarized in an upper direction is formed by heating the surface of the substrate 82 at a temperature of about 1050 °C after the surface of the substrate 82 is shield by a patterned mask according to a lift-off process. Each of non-inverted polarization layers 85 is positioned between the inverted polarization layers 84 as shown in Fig. 16B. Fig. 17 is a constitutional view of an optical system for examining optical characteristics of the wavelength changing device 81. As shown in Fig. 17, an optical system 87 is provided with a semiconductor laser 88 for radiating fundamental waves P1 of which the wavelength is variable, a converging system 89 for converging the fundamental waves P1, and the wavelength changing device 81. In the above configuration, fundamental waves P1 radiated from the semiconductor laser 88 are converged at the incident end facet 83a of the optical waveguide 83 by the converging system 89, and the fundamental waves P1 transmits through alternate rows of the inverted-polarization and non-inverted polarization layers 84, 85. At this time, the fundamental waves P1 are changed to second harmonic waves P2 of which a wavelength λ h is half of a wavelength λ f of the fundamental waves P1 in the inverted-polarization layer 84. Thereafter, the phase of the second harmonic waves P2 is inverted while transmitting through the inverted-polarization layer 84. Thereafter, the second harmonic waves P2 transmit through the non-inverted polarization layer 85. In this case, because the polarization direction of the non-inverted polarization layer 85 is opposite to the inverted-polarization layer 84, the second harmonic waves P2 transmitting through the non-inverted polarization layer 85 are amplified without attenuating. Therefore, a part of the fundamental waves P1 are changed to the second harmonic waves P2, and the second harmonic waves P2 are amplified in the optical waveguide 83 on condition that the quasi-phase condition indicated by the equation Λ2 = λ f /(2*(N2ω-Nω)) is satisfied. Here the symbol N2ω is a refractive index of the optical waveguide 83 for the second harmonic waves P2 and the symbol Nω is a refractive index of the optical waveguide 83 for the fundamental waves P1. In cases where the wavelength λ f of the fundamental waves P1 is 860 nm, the quasi-phase condition is satisfied. Thereafter, the second harmonic waves P2 are radiated from the output end facet 83b of the optical waveguide 83. Accordingly, when the pumping power of the fundamental waves P1 radiated from the semiconductor laser 88 is 70 mW to couple the fundamental waves P1 to the optical waveguide 83 at a power of 42 mW (or a coupling efficiency is 60 %), the intensity of the second harmonic waves P2 radiated from the wavelength changing device 81 is 3 mW. In contrast, the fundamental waves P1 not changed to the second harmonic waves P2 are reflected in the periodic structure consisting of the gratings 54 and the covering layer 55 in the same manner as in the first embodiment. That is, in cases where the wavelength λ f of the fundamental waves P1 is 860 nm, the periodic structure functions as the distributed Bragg reflector because the DBR condition Λ1 = mλ f /2N (m=2) is satisfied. Thereafter, the fundamental waves P1 reflected is radiated from the incident end facet 83a and is fed back to the semiconductor laser 88. Therefore, the wavelength λ f of the fundamental waves P1 is fixed to 860 nm even though an ambient temperature or an injecting current to the semiconductor laser 88 fluctuates. Also, even though the fundamental waves P1 reflected at the incident end facet 83a of the optical waveguide 83 is fed back to the semiconductor laser 88, the wavelength λ f of the fundamental waves P1 is fixed to 860 nm. Accordingly, the intensity of the second harmonic waves P2 obtained in the wavelength changing device 81 can be stabilized regardless of the fluctuation of the ambient temperature or the injecting current to the semiconductor laser 88. The stabilization of the intensity of the second harmonic waves P2 is measured with the optical system 87. Fig. 18 graphically shows the relation between the intensity of the second harmonic waves P2 and a temperature of the semiconductor laser 88 depending on the ambient temperature. As shown in Fig. 18, even though a temperature of the semiconductor laser 88 is changed in a range from 10 to 30 °C, the fluctuation of the intensity of the second harmonic waves P2 is restrained within 5 % of a maximum intensity. Accordingly, because the gratings 54 are uniformly formed without any damage caused by reactive ions to reflect the fundamental waves P1 not changed to the second harmonic waves P2 at high reflection efficiency and at low guided wave loss in the periodic structure consisting of the gratings 54 and the cover layer 55, the second harmonic waves P2 can be stably obtained in the wavelength changing device 81 regardless of the fluctuation of the ambient temperature or the injecting current to the semiconductor laser 88. Also, because the satisfaction of the quasi-phase condition Λ2 = λ f /{2*(N2ω-Nω)} is achieved regardless of the satisfaction of the DBR condition Λ1 = mλ f /2N, the wavelength λ f of the fundamental waves P1 can be arbitrarily selected. In other words, the wavelength λ h (=λ f /2) of the second harmonic waves P2 can be arbitrarily selected. Next, a modification of the fourth embodiment is described. The depth of the optical waveguide 83 is shallowed to 1.8 µm to increase the overlapping of the fundamental waves P1 and the periodic structure consisting of the gratings 54 and the cover layer 55. In this case, as is described in the second embodiment, the reflection efficient (or the diffraction efficient) in the wavelength changing device 81 is enhanced. The stabilization of the intensity of the second harmonic waves P2 measured with the optical system 87 is shown in Fig. 19. As shown in Fig. 19, even though a temperature of the semiconductor laser 88 is changed in a wide range from 10 to 50 °C, the fluctuation of the intensity of the second harmonic waves P2 is restrained within 5 % of a maximum intensity. Accordingly, because the intensity of the 860 nm wavelength fundamental waves P1 fed back to the semiconductor laser 88 is increased, the wavelength of the fundamental waves P1 radiated from the semiconductor laser 88 can be reliably fixed even though the temperature of the semiconductor laser 88 fluctuates in a wide range from 10 to 50 °C. As a result, the range in which the second harmonic waves P2 is stably obtained in the wavelength changing device 81 can be widened. In contrast, the intensity of the second harmonic waves P2 is slightly decreased. In the fourth embodiment, the substrate 82 is made of pure LiTaO 3 material. However, the material of the substrate 82 is not limited to the pure LiTaO 3 material. That is, it is applicable that LiTaO 3 material doped with MgO, Nb, Nd, or the like be utilized to make the substrate 82. Also, it is applicable that LiNbO 3 material be utilized to make the substrate 82. In addition, because KTiOPO 4 is a highly nonlinear optical crystal material, it is preferred that KTiOPO 4 be utilized to make the substrate 82. In this case, because a refractive index of KTiOPO 4 is a small value of about 1.7, the fundamental waves P1 can be reflected at high efficiency by the periodic structure consisting of the gratings 54 and the cover layer 55. Also, the optical waveguide 83 is formed according to the proton-exchange process. However, it is applicable that the optical waveguide 83 be formed by diffusing Ti or Nb into the substrate 82. Also, it is applicable that the optical waveguide 83 be formed by injecting ions such as Ti, Nb or the like into the substrate 82. 5. FIFTH EMBODIMENT: A fifth embodiment is described with reference to Figs. 20 to 21. Fig. 20A is a diagonal perspective view of a wavelength changing device according to a fifth embodiment of the present invention. Fig. 20B is a cross-sectional view of the wavelength changing device shown in Fig. 20A. As shown in Fig. 20A, a wavelength changing device 91 comprises the LiTaO 3 substrate 82, an optical waveguide 92 extending in a central upper side of the substrate 82 for transmitting coherent light consisting of 860 nm wavelength fundamental waves P1 from an incident end facet 92a to an output end facet 92b, a plurality of inverted-polarization layers 93 arranged in an upper side of the substrate 82 at second regular intervals Λ2 to cross the optical waveguide 92, the gratings 54 periodically arranged on the optical waveguide 92 which is positioned in the neighborhood of the incident end facet 92a (called a wave reflecting region 94), and the covering layer 55. The optical waveguide 92 is formed at a depth of 2 µm by exchanging a part of Li + ions of the substrate 82 for H + ions. Therefore, an effective refractive index of the optical waveguide 92 is slightly higher than that of the substrate 82 to confine large parts of the fundamental waves P1 in the optical waveguide 92. A width of the optical waveguide 92 is 4 µm. The inverted-polarization layers 93 dielectrically polarized in an upper direction is formed in the same manner as the inverted-polarization layers 84. The inverted-polarization layers 93 is not arranged in the neighborhood of the incident end facet 92a but arranged in the neighborhood of the output end facet 92b (called a wavelength changing region 95). Each of non-inverted polarization layers 96 is positioned between the inverted polarization layers 93 as shown in Fig. 20B. A total length of a series of gratings 54 in a propagation direction of the fundamental waves P1 is 1mm, and the periodic structure of the gratings 54 and the covering layer 55 is formed in a first grating order of the DBR periodic structure. That is, Λ1 = λ f /(2N) is satisfied. Here the symbol Λ1 (=0.2 µm) is regular intervals of the gratings 54, the wavelength λ f of the fundamental waves is 860 nm, and the symbol N (=2.15) is an effective refractive index of the optical waveguide 92. In the above configuration, fundamental waves P1 radiated from the semiconductor laser 88 shown in Fig. 17 are converged at the incident end facet 92a of the optical waveguide 92 and transmits through the wave reflecting region 94. In this case, a first radiation mode for the fundamental waves P1 is generated in the wave reflecting region 94. Therefore, the fundamental waves P1 are reflected by the periodic structure consisting of the gratings 54 and the covering layer 55 without changing the fundamental waves P1 to second harmonic waves P2. Thereafter, the fundamental waves P1 not reflected are changed to the second harmonic waves P2 in the wavelength changing region 95. At this time, because no distributed Bragg reflector is provided in the wavelength changing region 95. any radiation mode for the fundamental waves P1 is not generated. Therefore, the fundamental waves P1 are not reflected any more. Thereafter, the second harmonic waves P2 is radiated from the output end facet 92b of the optical waveguide 92. The first radiation mode for the fundamental waves P1 generated in the wave reflecting region 94 is equivalent to a second radiation mode for second harmonic waves P2 because the wavelength λ h of the second harmonic waves P2 is half of that of the fundamental waves P1. Therefore, in cases where the second harmonic waves P2 exist, the second harmonic waves P2 couple to a first radiation mode for the second harmonic waves P2. Therefore, the second harmonic waves P2 are led into the substrate 82 and the outside so that a radiation loss of the second harmonic waves P2 is increased. However, because the second harmonic waves P2 are not generated in the wave reflecting region 94, any second harmonic wave P2 is not lost. Accordingly, because the wave reflecting region 94 is arranged in the front of the wavelength changing region 95, the second harmonic waves P2 do not transmit through the wave reflecting region 94. Therefore, the second harmonic waves P2 can be obtained at high efficiency in the wavelength changing device 81. Also, though the gratings 54 made of the photoresist material 57 function as an absorber, the second harmonic waves P2 are not absorbed by the gratings 54 because the wave reflecting region 94 is arranged in the front of the wavelength changing region 95. Also, because the satisfaction of the quasi-phase condition Λ2 = λ f /{2*(N2ω-Nω)} is achieved regardless of the satisfaction of the DBR condition Λ1 = mλ f /2N, the wavelength λ f of the fundamental waves P1 can be arbitrarily selected. In other words, the wavelength λ h of the second harmonic waves P2 can be arbitrarily selected. The influence of the positional relation between the regions 94, 95 on optical characteristics of a wavelength changing device is shown in Fig. 21. As shown in Fig. 21, in cases where the wave reflecting region 94 is arranged in the rear of the wavelength changing region 95, a radiation loss of the second harmonic waves P2 is increased to 50 % so that the intensity of the second harmonic waves P2 radiated from the output end facet 92b is decreased to 3 mW. In contrast, in cases where the wave reflecting region 94 is arranged in the front of the wavelength changing region 95 according to the fifth embodiment, no radiation loss of the second harmonic waves P2 is obtained so that the intensity of the second harmonic waves P2 radiated from the output end facet 92b is increased to 5 mW. Also, when the pumping power of the fundamental waves P1 radiated from the semiconductor laser 88 is 60 mW to couple the fundamental waves P1 to the optical waveguide 92 at a power of 40 mW (or a coupling efficiency is 66 %), a reflection efficiency for the fundamental waves P1 is 15 % so that the wavelength of the fundamental waves P1 is completely fixed to 860 nm. Next, a modification of the fifth embodiment is described. The depth of the optical waveguide 92 is shallowed to 1.8 µm to increase the overlapping of the fundamental waves P1 and the periodic structure consisting of the gratings 54 and the cover layer 55. In this case, as is described in the second embodiment, the reflection efficiency (or the diffraction efficiency) in the wavelength changing device 81 is enhanced to 25 %. Also, the stabilization of the intensity of the second harmonic waves P2 is restrained within 3 % of a maximum intensity even though a temperature of the semiconductor laser 88 is changed in a wide range from 10 to 50 °C. Accordingly, the second harmonic waves P2 can be stably obtained at high efficiency. 6. SIXTH EMBODIMENT: A sixth embodiment is described with reference to Figs. 22 to 23. Fig. 22 is a cross-sectional view of a wavelength changing device according to a sixth embodiment. As shown in Fig. 22, a wavelength changing device 97 comprises the LiTaO 3 substrate 82, the optical waveguide 92 having a depth of 2 µm, a plurality of inverted-polarization layers 98 arranged in an upper side of the substrate 82 at second regular intervals Λ2, the gratings 54 periodically arranged on the optical waveguide 92 which is positioned in the neighborhood of the incident end facet 92a, and the covering layer 55. The inverted-polarization layers 98 dielectrically polarized in au upper direction is formed in the same manner as the inverted-polarization layers 84. The inverted-polarization layers 98 is not arranged in the neighborhood of the incident end facet 92a but arranged in the neighborhood of the output end facet 92b (called a wavelength changing region 99). A total length of a series of gratings 54 in a propagation direction of the fundamental waves P1 is 1mm, and the periodic structure of the gratings 54 and the covering layer 55 is formed in a second grating order of the DBR periodic structure. That is, the DBR condition Λ1 = 2*λ f /(2N) is satisfied. Here the symbol Λ1 (=0.4 µm) is first regular intervals of the gratings 54, the wavelength λ f of the fundamental waves is 860 nm, and the symbol N (=2.15) is an effective refractive index of the optical waveguide 92. In this case, a ratio of a width W1 of the gratings 54 to the regular intervals Λ1 of the gratings 54 is set to 0.23. The ratio W1/Λ1 = 0.23 is determined according to following reason. A relation between the ratio W1/Λ1 and the radiation loss of the fundamental waves P1 and another relation between the ratio W1/Λ1 and the reflection efficiency for the fundamental waves P1 are shown in Fig. 23. Though the reflection efficiency for the fundamental waves P1 is maximized at a first ratio W1/Λ1 = 0.25 and a second ratio W1/Λ1 = 0.75, the radiation loss of the fundamental waves P1 is large in the range from the first ratio W1/Λ1 = 0.25 to the second ratio W1/Λ1 = 0.75. Therefore, in cases where the first ratio W1/Λ1 = 0.25 is adopted, the radiation loss of the fundamental waves P1 to be changed to the second harmonic waves P2 becomes large so that the intensity of the second harmonic waves P2 is considerably decreased. Therefore, the ratio W1/Λ1 = 0.23 is adopted to keep the intensity of the fundamental waves P1 reflected at over a minimum value and to keep the intensity of the fundamental waves P1 changed to the second harmonic waves P2 at over a minimum value. In the above configuration, 860 nm wavelength fundamental waves P1 radiated from the semiconductor laser 88 shown in Fig. 17 are radiated to the optical waveguide 92. Therefore, first parts of the fundamental waves P1 are fed back to the semiconductor 88, second parts of the fundamental waves P1 are changed to second harmonic waves, and remaining parts of the fundamental waves P1 are lost, in the same manner as in the fifth embodiment. Optical characteristics of the wavelength changing device 97 are as follows. When the pumping power of the fundamental waves P1 radiated from the semiconductor laser 88 is 70 mW to couple the fundamental waves P1 to the optical waveguide 92 at a power of 42 mW (or a coupling efficiency is 60 %), a reflection efficiency for the fundamental waves P1 is 20 %. and a radiation loss of the fundamental waves P1 is 5 %. Also, the intensity of the harmonic waves P2 is 3 mW. Accordingly, even though the periodic structure of the gratings 54 and the covering layer 55 is formed in the second grating order, the fundamental waves P1 can be reflected at high efficiency, and the second harmonic waves P2 can be obtained at high efficiency. Also, because the periodic structure formed in the second grating order is easily manufactured as compared with that formed in the first grating order, the wavelength changing device 97 can be easily manufactured. Also, because the satisfaction of the quasi-phase condition Λ2 = λ f /{2*(N2ω-Nω)} is achieved regardless of the satisfaction of the DBR condition Λ1 = mλ f /2N, the wavelength λ f of the fundamental waves P1 can be arbitrarily selected. In other words, the wavelength λ h of the second harmonic waves P2 can be arbitrarily selected. The ratio W1/Λ1 of the periodic structure is not limited to 0.23. That is, it is applicable that the ratio W1/Λ1 of the periodic structure range from 0.05 to 0.24. Also, it is applicable that the ratio W1/Λ1 of the periodic structure range from 0.76 to 0.95. Because shorter wavelength laser light is stably obtained by changing the fundamental waves P1 to the harmonic waves P2 in the wavelength changing device 97, a shorter wavelength laser beam generating apparatus in which a shorter wavelength laser beam is generated at high power can be manufactured in a small size. Therefore, the apparatus can be utilized as a laser beam source of a photo disk and a laser printer. That is, a storage capacity of the photo disk can be greatly increased, and a small sized photo disk apparatus can he manufactured. 7. SEVENTH EMBODIMENT: A seventh embodiment is described with reference to Fig. 24. When a driving current supplied to a semiconductor laser or an ambient temperature varies, a refractive index of material of the semiconductor laser also varies. As a result, a wavelength of coherent light consisting of fundamental waves P1 radiated from the semiconductor laser generally changes. For example, a first driving current supplied to the semiconductor laser to read information stored in an optical disk greatly differs from a second driving current supplied to the semiconductor laser to write information in the optical disk. Therefore, a focal point of an objective lens utilized to converge the coherent light at the optical disk conventionally changes each time a reading operation and a writing operation is exchanged to each other. To avoid adverse influence of the change in the focal point, it is required to adjust the focal point. In the present invention, because the coherent light radiated from the semiconductor laser is fed back to the semiconductor laser by the function of the diffracting device 51, 71, 73, or 76 or the wavelength changing device 81, 91, 97, 101, 106, or 108, the wavelength of the coherent light radiated from the semiconductor laser is fixed. Therefore, even though the driving current or the ambient temperature varies, the focal point does not change. Therefore, the exchange between the reading operation and the writing operation can be quickly performed without any adjustment of the focal point. Accordingly, lens material having a large wavelength dispersion coefficient can be utilized in a laser beam generating apparatus. Also, a lens having a large numerical aperture (NA) can be utilized. Fig. 24 is a constitutional view of a shorter wavelength laser beam generating apparatus according to a seventh embodiment of the present invention. As shown in Fig. 24, a shorter wavelength laser beam generating apparatus 121 comprises a semiconductor laser 122 for radiating fundamental waves P1 having 0.8 µm wavelength band, a collimator lens 123 for collimating the fundamental waves P1, a focus lens 124 for focusing the fundamental wave P1 collimated, a wavelength changing device 81, 91, 97, 101, 106, or 108 for changing the fundamental waves P1 to second harmonic waves P2, a collimator lens 125 for collimating the second harmonic waves P2, a beam splitter 126 for splitting a beam of second harmonic waves P2 in two beams, a detector 127 for detecting a wavelength λ h of the second harmonic waves P2 splitted by the beam splitter 126, a plate type of heat insulator 128 made of quartz for thermally insulating the wavelength changing device 81. 91, 97, 101, 106, or 108 from the ambient air, and a box type of housing 129 for mounting the semiconductor laser 122, the converging optical system 123, 124, 125, and 126, the wavelength changing device, the detector 127, and the heat insulator 128. In the above configuration, fundamental waves P1 transmitting through the lenses 123, 124 are converged at the optical waveguide 83, or 92 of the wavelength changing device. Thereafter, 860 nm wavelength fundamental waves P1 selectively reflected by the wavelength changing device are fed back to the semiconductor laser 122. Therefore, the wavelength λ f of the fundamental waves P2 radiated from the semiconductor laser 122 is fixed to 860 nm, and the wavelength λ h of second harmonic waves P2 changed in the optical waveguide 83, or 92 is set to 430 nm. Thereafter, the second harmonic waves P2 are collimated by the collimator 125 and are splitted by the beam splitter 126. One of beams of waves P2 is output, and another beam transmits to the detector 127 to detect the wavelength λ h of the second harmonic waves P2. Accordingly, because the wavelength λ f of the fundamental waves P1 radiated from the semiconductor laser 122 is fixed to 860 nm, the wavelength λ f of the fundamental waves P1 can be prevented from fluctuating even though an ambient temperature of the semiconductor laser 122 or a driving current supplied to the semiconductor laser 122 fluctuates. Therefore, the output power of the second harmonic waves P2 can be stabilized, and noises included in the second harmonic waves P2 can be reduced. The second harmonic waves P2 is output at a high value of 2 mW. Also, because all parts of the shorter wavelength laser beam generating apparatus 121 are packed in the housing 129. the apparatus 121 can be manufactured in a small size. Therefore, the apparatus 121 can be useful to greatly increase the storage capacity of an optical disk and to manufacture the optical disk in a small size. 8. EIGHT EMBODIMENT: An eight embodiment is described with reference to Figs. 25 to 26. Fig. 25 is a diagonal view of a shorter wavelength laser beam generating apparatus according to a eleventh embodiment of the present invention. As shown in Fig. 25, a shorter wavelength laser beam generating apparatus 131 comprises the semiconductor laser 122 and a wavelength changing device 132. The device 132 comprises the substrate 82, the optical waveguide 83, the inverted-polarization layers 109 periodically arranged at the first regular intervals Λ1, the first electrode 110a, the second electrode 110b, and the electric source 111. The depth of the optical waveguide 83 is 2 µm, and the width of the optical waveguide 83 is 4 µm. A width W1 of each inverted-polarization layer 109 is set to 1.7 µm. The regular intervals Λ1 of the inverted-polarization layers 109 is 1.98 µm, and a total length of a series of layers 109 is 5 mm in a propagation direction of the fundamental waves P1. In the above configuration, when an electric potential is applied to the first electrode 110a, a periodic distribution of the refractive index is formed by a periodic structure consisting of the inverted and non-inverted polarization layers 109, 112 according to the electro-optic effect. Therefore, a diffraction grating is formed by the periodic distribution and functions as the distributed Bragg reflector on condition that the DBR condition is satisfied. Therefore, when the electric potential Vl(t) applied to the first electrode 110a is modulated, the wavelength λ f of the fundamental waves P1 reflected by the periodic structure is controlled because the averaged variation ΔN(V1) = {Δn*Λ1-W1) - Δn*W1}/Λ of the refractive index in the periodic structure is increased or decreased. Therefore, the wavelength λ f of the fundamental waves P1 radiated from the semiconductor laser 122 is controlled, as shown in Fig. 26. Accordingly, because the wavelength λ f of the fundamental waves P1 radiated from the semiconductor laser 122 is fixed depending on the periodic structure, the output power of the second harmonic waves P2 can be stabilized, and noise included in the second harmonic waves P2 can be reduced. In the eighth embodiment, the ratio of the width W1 to the regular intervals Λ1 is not equal to 0.5 to change the averaged variation ΔN. However, in cases where the depth of the inverted-polarization layers 109 is smaller than that of the optical waveguide 83, the wavelength λ f of the fundamental waves P1 reflected in the periodic structure can be controlled even though the ratio W1/Λ1 is equal to 0.5. That is, the intensity of the harmonic waves P2 can be modulated. 9. NINTH EMBODIMENT: A ninth embodiment is described with reference to Figs. 27 to 29. Fig. 27 is a cross-sectional view of a laser beam generating apparatus according to a ninth embodiment of the present invention. As shown in Fig. 27, a laser beam generating apparatus 133 comprises a Si sub-mount 134 having a length of 4 mm, the semiconductor laser 122 having an active layer 122a which is mounted on the sub-mount 134, and a diffracting device 135 mounted upside down on the sub-mount 134. The diffracting device 135 comprises the LiTaO 3 substrate 52, an optical waveguide 136 having an incident taper region 137 which is arranged in an upper side of the substrate 52, a plurality of gratings 138 periodically arranged at regular intervals Λ1 on the optical waveguide 136, and a passivation film 140 for covering the gratings 138. The active layer 122a is positioned at the same height on the sub-mount 134 as that of the incident taper region 137, and the semiconductor laser 122 is arranged closed to the incident taper region 137. Therefore, coherent light radiated from the active layer 122a of the semiconductor laser 122 is coupled to the incident taper region 137 of the optical waveguide 136 at a short time without transmitting through any lens. The gratings 138 is made of Ta 2 O 5 , and the regular intervals Λ1 of the gratings 138 are 1.9 µm to satisfy the DBR condition Λ1=m*λ c /(2N). Here the grating number m is 10, the wavelength λ c of the coherent light is 840 nm, and the refractive index N of the optical waveguide 144 is 2.2. In the above configuration, a part of the coherent light coupled to the incident taper region 137 is reflected by the gratings 138 and is fed back to the active layer 122a of the semiconductor laser 122. In contrast, a remaining part of the coherent light is output from an output end facet 136a of the optical waveguide 136. The wavelength λ c (=840 nm) of the coherent light reflected by the gratings 138 is determined to satisfy the DBR condition Λ1=10*λ c /(2N) which depends on the regular interval Λ1(= 1.9 µm) of the gratings 138 and the effective refractive index N(=2.2) of the optical waveguide 136. Therefore, the wavelength λ c of the coherent light radiated from the semiconductor laser 122 is fixed. Next, a manufacturing method of the diffracting device 135 and the laser beam generating apparatus 133 is described with reference to Figs. 28A to 28C. Figs. 28A to 28C are cross-sectional views showing a manufacturing method of the diffracting device 135 shown in Fig. 27. The optical waveguide 136 is manufactured by immersing the substrate 52 in a pyrophosphoric acid (H 4 P 2 O 7 ) solution according to the proton-exchange process. Therefore, the coherent light can be efficiently confined in the optical waveguide 136. Also, even though the optical waveguide 136 is optically damaged, a transmission loss of the coherent light can remain lowered. As shown in Fig. 28A, Ta is deposited on the entire surface of the substrate 52 at a thickness of 20 nm according to a sputtering process. Thereafter, a Ta film deposited is patterned to form slit shaped opening according to a photolithography process and a dry etching process. Thereafter, to form the incident taper region 137. one side of the substrate 52 is immersed in a pyrophosphoric acid (H 4 P 2 O 7 ) solution for thirty minutes at a temperature of 260 °C to exchange a part of Li + ions of the LiTaO 3 substrate 52 for H + ions, according to a proton-exchange process. Therefore, a proton-exchange layer having a thickness of 1.2 µm is formed in an upper side of the substrate 52 positioned just under the slit shaped opening. Thereafter, the substrate 52 is thermally processed for twenty minutes at a temperature of 420 °C, so that the incident taper region 137 having a thickness of 5 µm is formed. Thereafter, to form the optical waveguide 136, the other side of the substrate 52 is immersed in the pyrophosphoric acid (H 4 P 2 O 7 ) solution for twelve minutes at a temperature of 260 °C to exchange a part of Li + ions of the LiTaO 3 substrate 52 for H + ions, according to the proton-exchange process. Therefore, another proton-exchange layer having a thickness of 0.5 µm is formed in another upper side of the substrate 52 positioned just under the slit shaped opening. Thereafter, the substrate 52 is thermally processed for one minute at a temperature of 420 °C, so that the optical waveguide having a thickness of 1.9 µm is formed. Thereafter, as shown in Fig. 28B, Ta 2 O 5 is coated over both the substrate 52 and the optical waveguide 136 to form a Ta 2 O 5 film 139 having a thickness of 30 nm. Thereafter, as shown in Fig. 38C, the Ta 2 O 5 film 139 is etched in a periodic pattern according to a photolithography process and a dry etching process to form the gratings 138. The regular intervals Λ1 of the gratings 138 are 1.9 µm, so that the grating number becomes m=10. Thereafter, SiO 2 is deposited on the gratings 138 according to a sputtering process to form the passivation film 140 having a thickness of 2 µm. The height of the incident taper region 137 and the height of the active layer 122a becomes the same as each other by adjusting the thickness of the passivation film 140. Thereafter, both sides of the optical waveguide 136 are polished. The length of the optical waveguide 136 is 3 mm. Thereafter, the semiconductor laser 122 is bonded upside down to the sub-mount 134. Thereafter, the diffracting device 135 is mounted on the sub-mount 134 while attaching the passivation film 140 to the sub-mount 134, and the diffracting device 135 is strictly positioned and bonded to the sub-mount 134 while the semiconductor laser 122 radiates the coherent light so as to maximize the intensity of the coherent light radiated to the incident taper region 137 of the optical waveguide 136. As a result, the device 133 is completely manufactured. Accordingly, because any optical system such as a converging lens or a collimator lens is not utilized, the device 133 can be manufactured in a small size. Next, optical characteristics of the laser beam generating apparatus 133 is described. The reflection efficiency of the gratings 138 is only 10 %. Though the reflection efficiency is not high, the value 10 % is enough to stably fix the wavelength of the coherent light radiated from the semiconductor laser 122 because the semiconductor laser 122 is arranged close to the diffracting device 135. Also, even though the intensity of the coherent light radiated from the semiconductor laser 122 is modulated, the wavelength of the coherent light radiated from the semiconductor laser 122 is stably fixed because the coherent light radiated from the semiconductor laser 122 is reflected and fed back to the semiconductor laser 122 at a short time. Fig. 29 graphically shows relation between a driving current supplied to the semiconductor laser 122 and the wavelength λ c of the coherent light radiated from the semiconductor laser 122. As shown in Fig. 29, the wavelength λ c of the coherent light conventionally varies by 5 nm when the driving current changes by 50 mA. However, no variation of the wavelength λ c of the coherent light is observed in the device 133 according to the present invention. Accordingly, because the wavelength λ c of the coherent light is stably fixed, the coherent light obtained in the device 133 can be useful. In the ninth embodiment, the gratings 138 made of Ta 2 O 5 is formed by etching the Ta 2 O 5 film 139 according to the photolithography process and the dry etching process. However, the material of the gratings 138 is not limited to Ta 2 O 5 . Also, the manufacturing method of the gratings 138 is not limited to the dry etching process. For example, the gratings 54 shown in Fig. 8A is appliable. 10. TENTH EMBODIMENT: A tenth embodiment is described with reference to Figs. 30, 31. Fig. 30 is a constitutional view of a laser beam generating apparatus according to a tenth embodiment of the present invention. As shown in Fig. 30, a laser beam generating apparatus 141 comprises the Si sub-mount 134, the semiconductor laser 122, and a diffracting device 142 mounted upside down on the sub-mount 134. Fig. 31A is a diagonal perspective view of the diffracting device 142 shown in Fig. 30. Fig. 31B is a cross-sectional view of the diffracting device 142 shown in Fig. 31A. As shown in Figs. 31A, 31B, the diffracting device 142 comprises a LiNbO 3 substrate 143, an optical waveguide 144 having an incident taper region 144a which is arranged in an upper side of the substrate 143, a plurality of gratings 145 periodically arranged at regular intervals Λ1 on the optical waveguide 144, a dielectric passivation film 146 for protecting and insulating the optical waveguide 144 and the gratings 145 from the outside, a first electrode 147 arranged just over the optical waveguide 144 through the gratings 145 and the dielectric passivation film 146 for inducing electric field which penetrates through the optical waveguide 144, and a second electrode 148 arranged over the substrate 143 through the dielectric passivation film 146. The substrate 143 is formed by cutting out LiNbO 3 crystal in a direction perpendicular to a Z-axis defined as [001]-direction in Miller indices. Therefore, the LiNbO 3 substrate 143 (or +Z plate) has an upper surface defined as (001)-plane in Miller indices. The optical waveguide 144 is formed at a length of 10 film by exchanging a part of Li + ions of the substrate 143 for H + ions. Therefore, an effective refractive index of the optical waveguide 144 is slightly higher than that of the substrate 143 to confine large parts of coherent light in the optical waveguide 144. Also, because the electro-optic effect on LiNbO 3 is very large, the refractive index of the optical waveguide 144 greatly changes by inducing electric field in the optical waveguide 144. A thickness of the optical waveguide 144 is set close to a cut-off thickness which is equivalent to a minimum thickness required to transmit coherent light through the optical waveguide 144. Therefore, in cases where no electric field penetrates through the optical waveguide 144, the coherent light can transmit through the optical waveguide 144. In contrast, in cases where electric field penetrates through the optical waveguide 144 to decrease the refractive index of the optical waveguide 144, the coherent light cannot transmit through the optical waveguide 144. That is, the coherent light is cut off in the optical waveguide 144. The gratings 145 is made of Ta 2 O 5 , and the regular intervals Λ1 of the gratings 145 are 0.19 µm to satisfy the DBR condition Λ1=m*λ c /(2N). Here the wavelength λ c of the coherent light is 840 nm and the refractive index N of the optical waveguide 144 is 2.2. Therefore, the grating number m becomes equal to 1, so that the coherent light transmits through the optical waveguide 144 in a single mode. The dielectric passivation film 146 is made of SiO 2 , and the film 146 prevents the first electrode 147 made of metal from being directly in contact with the optical waveguide 144. Therefore, the transmission loss of the optical waveguide 144 for the coherent light is greatly decreased. The first electrode 147 has a width of 4 µm and a thickness of 200 nm, and a positive electric potential is applied to the first electrode 147 to decrease the refractive index of the optical waveguide 144. The second electrode 148 is grounded. Also, the distance between the electrodes 146, 147 is 5µm. The height of the active layer 122a is the same as that of the incident taper region 144a, and the semiconductor laser 122 is arranged closed to the incident taper region 144a. Therefore, coherent light radiated from the active layer 122a of the semiconductor laser 122 is coupled to the incident taper region 144a of the optical waveguide 144 at a short time without transmitting though any lens. In the above configuration, in cases where no electric potential is applied to the first electrode 147, the coherent light coupled to an incident end facet 144b transmits through the optical waveguide 144 in a single mode, and a part of the coherent light is reflected by the gratings 145 to be fed back to the active layer 122a of the semiconductor laser 122. Also. a remaining part of the coherent light is output from an output end facet 144c of the optical waveguide 144. Therefore, the wavelength λ c of the coherent light radiated from the semiconductor laser 122 is fixed. In contrast, in cases where a positive electric potential is applied to the first electrode 147, electric field is induced in the optical waveguide 144 to decrease the refractive index of the optical waveguide 144. Therefore, the coherent light coupled to the incident taper region 144b is cut off and transmits to the substrate 134 because the coherent light is coupled to a first radiation mode. As a result, no coherent light is output from the output end facet 144c of the optical waveguide 144. For example, when the electric potential V1 applied to the first electrode 147 is 10 V, the intensity of the electric field becomes 2 × 10 6 V/m so that the refractive index of the optical waveguide 144 decreases by 10 -4 . Therefore, the coherent light is cut off. Also, when a pulsated electric potential having a peak voltage 10 V is repeatedly applied to the first electrode 147 at a cycle of 2 ns (a frequency of 500 MHz), the intensity of the coherent light output from the optical waveguide 144 is modulated at a frequency of 500 MHz, and the wavelength λ c of the coherent light is stably fixed to 840 nm. Accordingly, the coherent light modulated and fixed at a prescribed wavelength can be reliably obtained. In the tenth embodiment, the substrate 134 is made of LiNbO 3 because the electro-optic effect on LiNbO 3 is very large. However, the material of the substrate 134 is not limited to LiNbO 3 . For example, a ferroelectric substance such as LiTaO 3 can be appliable. Also, the gratings 145 made of Ta 2 O 5 is formed by etching the Ta 2 O 5 film 139 according to the photolithography process and the dry etching process. However, the material of the gratings 145 is not limited to Ta 2 O 5 . Also, the manufacturing method of the gratings 145 is not limited to the dry etching process. For example, the gratings 54 shown in Fig. 8A is appliable. 11. ELEVENTH EMBODIMENT: An eleventh embodiment is described with reference to Fig. 32. Fig. 32 is a constitutional view of an optical information processing apparatus according to a fourteenth embodiment of the present invention. As shown in Fig. 32, an optical information processing apparatus 151 comprises the laser beam generating apparatus 133 of 6 mm square, a collimator lens 152 for collimating coherent light radiated from the device 133, a beam splitter 153 for splitting a beam of coherent light collimated by the lens 152 in two beams, an objective lens 154 for converging the coherent light splitted by the lens 152 at an optical disk 155 in which information is stored, a converging lens 156 for converging the coherent light which is reflected by the optical disk 155 and is splitted by the beam splitter 153, and a Si detector 157 for detecting the intensity of the coherent light converged by the converging lens 156. The objective lens 154 is made of SF8 (manufactured by HOYA glass LTD. in Japan) having a refractive index of 1.68, and a numerical aperture NA of the lens 154 is 0.6. In the above configuration, coherent light P1 radiated from the semiconductor laser 122 transmits through the optical waveguide 136 at a transverse magnetic (TM oo ) mode equivalent to a lowest-order mode. Thereafter, the coherent light P1 radiated from the device 133 is radiated to the optical disk 155 to form a converging spot of 1.1 µm in diameter after the coherent light P1 transmits through the collimator lens 152, the beam splitter 153, and the objective lens 154. Thereafter, reflected coherent light is detected by the detector 157 after the reflected coherent light transmits through the objective lens 154, the beam splitter 153, and the converging lens 156. Because the wavelength of the coherent light is stably fixed in the device 133, a beam of coherent light can be stably radiated to a desired pit of the optical disk 155. Therefore, even though information is stored in the optical disk 155 with high density, the information can be read with high accuracy. For example, a relative intensity of noise (RIN) to an information signal is -140 dB/Hz. In the eleventh embodiment, the objective lens 154 is made of SF8. However, the material of the objective lens 154 is not limited to SF8. For example, even though SF6 having high refractive index and high dispersion coefficient is applied as the material of the objective lens 154, the information can be still read with high accuracy because the wavelength of the coherent light is stably fixed in the device 133. Also, the apparatus 151 can be applied to write information in the optical disk 155. 12. TWELFTH EMBODIMENT: A twelfth embodiment is described with reference to Figs. 33, 34. Fig. 33 is a cross-sectional view of a shorter wavelength laser beam generating apparatus according to a twelfth embodiment of the present invention. As shown in Fig. 33, a shorter wavelength laser beam generating apparatus 161 comprises the Si sub-mount 134 of 10 mm square, the semiconductor laser 122, and a wavelength changing device 162 mounted upside down on the sub-mount 134. The wavelength changing device 162 comprises the LiTaO 3 substrate 82, an optical waveguide 163 having an incident taper region 164 which is arranged in an upper side of the substrate 82, a plurality of gratings 138 periodically arranged at grating intervals Λ1 on the optical waveguide 163, the inverted-polarization layers 84 periodically arranged at matching intervals Λ2 in the upper side of the substrate 82 to cross the optical waveguide 163, and the passivation film 140 for covering the gratings. The height of the active layer 122a is the same as that of the incident taper region 164, and the semiconductor laser 122 is arranged closed to the incident taper region 164. Therefore, coherent light radiated from the active layer 122a of the semiconductor laser 122 is coupled to the incident taper region 164 of the optical waveguide 163 at a short time without transmitting though any lens. The optical waveguide 163 including the incident taper region 164 is manufactured in the same manner as the optical waveguide 136. Fig. 34 is a constitutional view of an optical information processing apparatus according to a twelfth embodiment of the present invention. As shown in Fig. 34, an optical information processing apparatus 165 comprises the shorter wavelength laser beam generating apparatus 161 of 10 mm square, the collimator lens 152, the beam splitter 153, the objective lens 154, the converging lens 156, and the Si detector 157. In the above configuration, a second harmonic wave P2 is radiated to a desired pit in the optical disk 155 to form a converging spot of 0.6 µm in diameter. The diameter of 0.6 µm is smaller than a conventional diameter 0.78 µm obtained in a conventional apparatus. Therefore, information stored with higher density more than that in the apparatus 151 can be read or written. Also, a relative intensity of noise (RIN) to an information signal is -145 dB/Hz. Therefore, the information signal can be clearly obtained. 13. THIRTEENTH EMBODIMENT: A thirteenth embodiment is described with reference to Fig. 35. Fig. 35 is a cross-sectional view of a laser beam generating apparatus according to a thirteenth embodiment of the present invention. As shown in Fig. 35, a laser beam generating apparatus 171 comprises the Si sub-mount 134 of 6 mm square, a semiconductor laser 172 having an active layer 172a for radiating coherent light having a wavelength of 1.552 µm, and a diffracting device 173 mounted upside down on the sub-mount 134. The diffracting device 173 comprises a substrate 174 made of glass, the optical waveguide 136, the gratings 138, and the passivation film 140 for covering the gratings 138. The height of the active layer 172a is the same as that of the incident taper region 137, and the semiconductor laser 172 is arranged closed to the incident taper region 137. Therefore, coherent light radiated from the active layer 172a of the semiconductor laser 172 is coupled to the incident taper region 137 of the optical waveguide 136 at a short time without transmitting though any lens. In the above configuration, coherent light having a wavelength of 1.552 µm is stably obtained in a temperature range from 0 to 50 °C. Therefore, even though an ambient temperature fluctuates, the coherent light having the wavelength of 1.552 µm can be stably obtained. In the laser beam generating apparatus 133, 141, 161, and 171, no lens is utilized to radiate light to the optical waveguide 136, 144, or 163. However, it is appliable that a lens is arranged between the semiconductor laser 122 or 172 and the device 135, 142, 162, or 173. Also, the material of the sub-mount 134 is not limited to Si. For example, material having high thermal conductivity such as Cu or C is useful for the sub-mount 134. Also, the substrates 52, 82, 143, and 174 are respectively made of LiTaO 3 or LiNbO 3 . However, ferromagnetic material such as KNbO 3 and KTiOPO 4 and organic material such as MNA are appliable.

Claims: A diffracting device, comprising: a substrate (52,82); an optical waveguide (53,72,83,92) arranged in the substrate for transmitting coherent light in a propagation direction, wherein a first cross-sectional area of the optical waveguide positioned at a reflecting region is smaller than a second cross-sectional area of the optical waveguide positioned at a non-reflecting region; a plurality of resist elements (54) having a first refractive index N1 and periodically arranged adjacent to the optical waveguide of the reflecting region at regular intervals in the propagation direction of the coherent light transmitting through the optical waveguide, the resist elements being made of a soft material which has high workability; and a reflecting element (55,77) having a second refractive index N2 higher than the first refractive index N1 of the resist elements and arranged between the resist elements for reflecting the coherent light distributed in the reflecting element, a refractive change being made by a periodic structure consisting of the resist element and the reflecting element, and overlap between the coherent light and the reflecting element being, in use, increased in the reflecting region. A diffracting device according to claim 1, in which a ratio of a width of each resist element in the propagation direction of the coherent light to the regular interval Λ of the resist elements is in a first range of from 0.05 to 0.24 or in a second range of from 0.76 to 0.95 on condition that A = mλ/(2N), where m=2, the symbol λ is the wavelength of the coherent light and the symbol N is the effective refractive index of the optical waveguide. A wavelength changing device comprising a diffracting device according to claim 1 in which the substrate is made of a non-linear crystal polarized in a first polarization direction perpendicular to the propagation direction, and the wavelength changing device further comprises a plurality of inverted polarization layers (84,93,98) polarized in a second polarization direction opposite to the first polarization direction of the substrate and periodically arranged in the substrate at phase-matching intervals in the propagation direction to periodically cross the optical waveguide, a part of fundamental waves of the coherent light being changed to second harmonic waves in alternate rows of the optical waveguide and the inverted polarization layers. A wavelength changing device according to claim 3 in which the regular interval Λ1 of the resist elements satisfies a distributed Bragg condition Λ1 = mλ f /(2N), where m=1 or 2, the symbol λ f is a wavelength of the fundamental waves and the symbol N is an effective refractive index of the optical waveguide, and the phase-matching interval Λ2 of the inverted polarization layers satisfies a quasi-phase matching condition Λ2 = λ f /{2(N2ω - Nω)} where the symbol N2ω is a refractive index of the alternate rows for the second harmonic waves and the symbol Nω is a refractive index of the alternate rows for the fundamental waves. A wavelength changing device according to claim 3 or 4 in which the resist elements are locally arranged in the neighbourhood of an incident side of the optical waveguide in which no second harmonic wave exists, and the inverted polarization layers are locally arranged in the neighbourhood of an output side of the optical waveguide through which the fundamental waves not changed to the second harmonic waves transmit. A wavelength changing device according to claim 3, 4 or 5 in which a ratio of a width of each resist element in the propagation direction of the coherent light to the regular interval Λ1 of the resist elements is in a first range of from 0.05 to 0.24 or in a second range of from of 0.76 to 0.95 on condition that an equation Λ1 = mλ f /(2N), where m=2, the symbol λ f is a wavelength of the fundamental waves and the symbol N is an effective refractive index of the optical waveguide. A method for manufacturing a diffracting device, comprising the steps of: forming an optical waveguide (53,72,83,92) in a substrate (52,82) such that a first cross-sectional area of the optical waveguide positioned at a reflecting region is smaller than a second cross-sectional area of the optical waveguide positioned at a non-reflecting region, coherent light, in use, transmitting through the first and second cross-sectional areas of the optical waveguide in a propagation direction; patterning a resist soft material having a high workability placed on the first cross-sectional area of the optical waveguide to form a plurality of resist elements (54) having a first refractive index N1 periodically arranged at regular intervals in the propagation direction of the coherent light, and arranging a reflecting element (55,77) having a second refractive index N2 higher than the first refractive index N1 of the resist elements between the resist elements to form a refractive change in a periodic structure consisting of the resist elements and the reflecting element and to reflect the coherent light distributed in the reflecting element in each of alternate rows of the resist elements and the reflecting element overlap between the coherent light and the reflecting element, in use, being increased in the reflecting region. A method according to claim 7 in which the step of patterning a resist soft material includes: preparing a photoresist material as the resist soft material; arranging the photoresist material on the optical waveguide to form a photoresist film; exposing periodic structure areas of the photoresist film to exposing light; developing the photoresist film exposed to the exposing light; and removing the photoresist film of the periodic structure areas to form the resist elements made of the photoresist material. A method according to claim 7 or 8 in which the regular interval Λ of the resist elements satisfies an equation Λ = mλ/(2N), where m=1 or 2, the symbol λ is a wavelength of the coherent light and the symbol N is an average refractive index of the periodic structure. A method according to claim 7, 8 or 9, further comprising the steps of: preparing the substrate made of a non-linear crystal polarized in a first polarization direction perpendicular to the propagation direction; and periodically arranging a plurality of inverted polarization layers, polarized in a second polarization direction opposite to the first polarization direction of the substrate, in the substrate at phase-matching intervals in the propagation direction to periodically cross the optical waveguide and change a part of fundamental waves of the coherent light to second harmonic waves in alternate rows of the optical waveguide and the inverted polarization layers. A method according to claim 10 in which the regular interval Λ1 of the resist elements satisfies a distributed Bragg condition Λ1 = mλ f /(2N), where m=1 or 2, the symbol λ f is a wavelength of the fundamental waves and th symbol N is an average refractive index of the periodic structure, and the phase-matching interval Λ2 of the inverted polarization layers satisfies a quasi-phase matching condition Λ2 = λ f /{2(N2ω - Nω)} where the symbol N2ω is a refractive index of the alternate rows for the second harmonic waves and the symbol Nω is a refractive index of the alternate rows of the fundamental waves. A method according to claim 7, 8, 9 or 10 in which the step of patterning a resist soft material includes: adjusting a ratio of a width of each resist element in the propagation direction of the coherent light to the regular interval of the resist elements so that it is in a first range of from 0.05 to 0.24 or in a second range of from 0.76 to 0.95. A method according to any one of claims 7 to 12 in which the reflecting element is made of a material selected from the group consisting to Ta 2 O 5 , TiO 2 , SiN, and burning type metallic oxide such as TiO 2 . A method according to any one of claims 7 to 13 in which the substrate is made of LiTa (1-X) Nb X O 3 (0 ≤ X ≤ 1). A method according to any one of claims 7 to 14 in which the optical waveguide is formed by exchanging a part of Li + ions included in the substrate for H + ions. A laser beam generating apparatus comprising a diffracting device according to claim 1, and further comprising: a semiconductor laser (88) for radiating a beam of coherent light consisting of fundamental waves to an incident end of the optical waveguide to reflect a part of the coherent light by the periodic structure toward the semiconductor laser and fix the wavelength of the coherent light radiated from the semiconductor laser, the coherent light radiated to the optical waveguide being output from an output end of the optical waveguide. A device or apparatus according to claim 1 or 16 in which the regular interval Λ of the resist elements satisfies an equation Λ = mλ/(2N), where m=1 or 2, the symbol λ is a wavelength of the coherent light and the symbol N is an effective refractive index of the optical waveguide. A laser beam generating apparatus according to claim 16 in which a ratio of a width of each resist element in the propagation direction of the coherent light to the regular interval Λ of the resist elements is in a first range from 0.05 to 0.24 or in a second range from 0.76 to 0.95 on condition that an equation Λ = mλ/(2N), where m=2, the symbol λ is a wavelength of the coherent light and the symbol N is an effective refractive index of the optical waveguide. A device or apparatus according to claim 1, 3 or 16 in which the reflecting element is made of a material selected from the group consisting of Ta 2 O 5 , TiO 2 and SiN. A laser beam generating apparatus according to claim 16 in which the semiconductor laser has an active layer from which the coherent light is radiated, the optical waveguide has an incident taper region positioned in the neighbourhood of the incident end, and the active layer of the semiconductor laser is arranged closely to the incident taper region of the optical waveguide at the same height as the incident taper region, the incident taper region being deeper than another region of the optical waveguide to receive the coherent light radiated from the active layer in the incident taper region. A laser beam generating apparatus according to claim 16 in which the substrate is made of a non-linear crystal polarized in a first polarization direction perpendicular to the propagation direction, and the laser beam generating apparatus, further comprising: a plurality of inverted polarization layers (84,93,98) polarized in a second polarization direction opposite to the first polarization direction of the substrate and periodically arranged in the substrate at phase-matching intervals in the propagation direction to periodically cross the optical waveguide, a part of fundamental waves of the coherent light being changed to second harmonic waves in alternate rows of the optical waveguide and the inverted polarization layers. A laser beam generating apparatus according to claim 21 in which the fundamental waves transmit through the optical waveguide in a single mode. A laser beam generating apparatus according to claim 21 or 22 in which the regular interval Λ of the inverted polarization layers satisfies an equation Λ = mλ/(2N), where m=1 or 2, the symbol λ is a wavelength of the coherent light and the symbol N is an effective refractive index of the optical waveguide. A laser beam generating apparatus according to claim 21, 22 or 23 in which a width Wa of the inverted polarization layer in the propagation direction and the regular interval Λ of the inverted polarization layers satisfy a relation Wa ≠ Λ - Wa. A laser beam generating apparatus according to claim 21, 22, 23 or 24 in which a depth Da of the inverted polarization layer in the first polarization direction and a depth Dw of the optical waveguide in the first polarization direction satisfy a relation Da < Dw. A laser beam generating apparatus according to any one of claims 21 to 25, further comprising: a plurality of second inverted polarization layers (109) polarized in the second polarization direction and periodically arranged in the substrate at second phase-matching intervals in the propagation direction to periodically cross the optical waveguide, a part of fundamental waves of the coherent light being changed to second harmonic waves in alternate rows of the optical waveguide and the second inverted polarization layers. An optical information processing apparatus for reading or writing information from/to an information medium, the apparatus comprising a diffracting device according to claim 1, and further comprising: a semiconductor laser (88) for radiating a beam of coherent light consisting of fundamental waves to an incident end of the optical waveguide to reflect a part of the coherent light by the periodic structure toward the semiconductor laser and fix the wavelength of the coherent light radiated from the semiconductor laser, the coherent light radiated to the optical waveguide being output from an output end of the optical waveguide; an optical system for converging the coherent light output from the output end of the optical waveguide at the information medium; and a detector for detecting intensity of the coherent light reflected by the information medium, the intensity of the coherent light depending on the information stored in the information medium. An optical information processing apparatus according to claim 27 in which the substrate is made of a non-linear crystal polarized in a first polarization direction perpendicular to the propagation direction, and the optical information processing apparatus, further comprising: a plurality of inverted polarization layers (84,93,98) polarized in a second polarization direction opposite to the first polarization direction of the substrate and periodically arranged in the substrate at phase-matching intervals in the propagation direction to periodically cross the optical waveguide, a part of the fundamental waves being changed to second harmonic waves in alternate rows of the optical waveguide and the inverted polarization layers, and the second harmonic waves being converged at the information medium and the intensity of the second harmonic waves being detected by the detector. A device or apparatus according to any one of claims 1 to 6 or 16 to 28 in which the substrate is made of LiTa (1-X) Nb X O 3 wherein 0 ≤ X ≤ 1. A device or apparatus according to any one of claims 1 to 6 or 16 to 29 in which the optical waveguide has a composition determined by exchanging a part of Li + ions included in the substrate for H + ions.

Inventors: MIZUUCHI KIMINORI, YAMAMOTO KAZUHISA, MIZUUCHI, KIMINORI, YAMAMOTO, KAZUHISA
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: G02B6
IPC codes: G02F1, H01S5, G02B6