Wavelength changing device and laser beam generating apparatus

Abstract: A wavelength changing device consists of a substrate made of LiTaO 3 , 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 inverted polarization layers in the substrate, a dielectric film and a plurality of gratings periodically arranged on the dielectric film at regular intervals. 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. A 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 wavelength changing device having an optical waveguide with periodically inverted-polarization layers and a laser beam generating apparatus in which a laser beam is generated with the wavelength changing 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 Al. 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 reprobability 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 reprobability 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", Optics 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 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", Apply. 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 A 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 T5 %. 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. An aim of the present invention is to provide a wavelength changing device in which a plurality of gratings periodically arranged are uniformly 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. Another aim of the present invention is to provide a I laser beam generating apparatus in which a laser beam having a fixed wavelength is stably generated with the wavelength changing device. According to the present invention there is provided a wavelength changing device, comprising: a substrate made of a non-linear crystal, the substrate being polarized in a first polarization direction; an optical waveguide arranged in the substrate for transmitting fundamental waves in a propagation direction perpendicular to the first polarization direction; and a plurality of inverted polarization layers polarized in a second polarization direction opposite to the first polarization direction of the substrate and periodically arranged in the substrate at regular intervals in the propagation direction to change a part of the fundamental waves to second harmonic waves in alternate rows of the optical waveguide and the inverted polarization layers periodically crossing the optical waveguide; a dielectric film arranged on the optical waveguide for confining the second harmonic waves produced in the alternate rows to prevent the second harmonic waves spreading outside the optical waveguide; and a plurality of grating elements periodically arranged on the dielectric film at grating intervals in the propagation direction of the fundamental waves for reflecting the remaining part of the fundamental waves spreading outside the dielectric film. In the above configuration, a part of the 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. Therefore, the fundamental waves not changing to the second harmonic waves and the second harmonic waves transmit through the optical waveguide on which the dielectric film and the gratings are arranged. An intensity distribution in a depth direction (parallel to the first direction) of the fundamental and second harmonic waves transmitting through the optical waveguide generally depends on the wavelength of the waves. That is, the spread of the fundamental waves in intensity distribution is larger than that of the second harmonic waves. Therefore, the fundamental waves spread out over the dielectric film. In contrast, the second harmonic waves are confined into the dielectric film and the optical waveguide. As a result, the fundamental waves are selectively reflected by the grating, and the fundamental waves are fed back to a fundamental wave source to fix the wavelength of the fundamental waves. Also, the second harmonic waves are output from an output end facet of the optical waveguide without being reflected by the gratings. Accordingly, the fundamental waves can be selectively reflected at high reflection efficiency in the wavelength changing device according to the present invention. Also, a transmission loss for the harmonic waves is lowered. The present invention also provides a laser beam generating apparatus, including the wavelength changing device described above and further comprising: a semiconductor laser for radiating a beam of the fundamental waves to an incident end of the optical waveguide, the fundamental waves being reflected by the grating elements toward the semiconductor laser to fix the wavelength of the fundamental waves radiated from the semiconductor laser. In the above configuration, the fundamental waves radiated from the semiconductor laser spread outside the optical waveguide and the dielectric film. Therefore, the fundamental waves are selectively reflected by the gratings to the semiconductor laser, and the wavelength of the fundamental waves radiated from the semiconductor laser are fixed. Therefore, even though a driving current supplied to the semiconductor laser or an ambient temperature varies, the coherent light having a fixed wavelength can be obtained in the laser beam generating apparatus. In contrast, the second harmonic waves produced in the alternate rows do not spread outside the dielectric film. Therefore, the second harmonic waves transmitting through the optical waveguide are not reduced by the gratings. Accordingly, the second harmonic waves can be efficiently obtained. 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 described for reference; 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 second diffracting device described for reference; 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 device of Fig. 13A; 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 described for reference; Fig. 16A is a diagonal perspective view of another wavelength changing device described for reference; 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 an embodiment of the present invention; Fig. 20B is a cross-sectional view of the wavelength changing device shown in Fig. 20A; Fig. 21 is an enlarged cross-sectional view of a wave reflecting region in the wavelength changing device shown in Fig. 20A, intensity distributions of fundamental waves P1 and second fundamental waves P2 being explanatorily shown; Fig. 22 graphically shows relation between the wavelength of fundamental waves P1 and regular intervals Λ1 of the gratings in the wavelength changing device shown in Fig. 20A; Fig. 23 graphically shows relation between the intensity of harmonic waves P2 and the thickness D1 of a dielectric passivation film in the wavelength changing device shown in Fig. 20A; and Fig. 24 is a constitutional view of a shorter wavelength laser beam generating apparatus according to a second embodiment of the present invention. Fig. 8A is a diagonal perspective view of a diffracting device described for reference. 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 um. 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 nm 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 nm 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 nm 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 Al of the gratings 54. Therefore, the reflection efficiency can be increased. In the reference device, 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. Fig. 13A is a diagonal perspective view of a second diffracting device described for reference. 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 coheren 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 um, 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 reference device, 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 hight 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. Fig. 15 is a diagonal perspective view of a wavelength changing device described for reference. 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%. Fig. 16A is a diagonal perspective view of another wavelength changing device described for reference. 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 reference device 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 descrebed 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 this reference device, 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 non-linear 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. An embodiment of the invention is described with reference to Figs. 20 to 23. Fig. 20A is a diagonal perspective view of a wavelength changing device according to an embodiment of the present invention. Fig. 20B is a cross-sectional view of the wavelength changing device shown in Fig. 20A. As shown in Figs. 20A, 20B, a wavelength changing device 101 comprises the substrate 82, the optical waveguide 83, the inverted-polarization layers 84 periodically arranged at the second regular intervals Λ2, a dielectric passivation film 102 deposited on the substrate 82 and the optical waveguide 83 for protecting the optical waveguide 83, and a plurality of gratings 103 periodically arranged on the dielectric passivation film 102 at the first regular intervals Λ1. The depth of the optical waveguide 83 is 2 µm, and the width of the optical waveguide 83 is 4 µm. Also, the second regular intervals Λ2 of the inverted-polarization layers 84 are about 3.6 µm. Therefore, when the wavelength λ f of the fundamenal waves P1 is 860 nm, the quasi-phase matching condition 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. The dielectric passivation film 102 is made of SiO 2 , and the thickness D1 of the film 102 is 0.06 µm. A refractive index of the dielectric passivation film 102 is 0.5. The gratings 103 are made of Ta 2 O 5 , and the size of the gratings 103 is the same as that of the gratings 54 shown in Fig. 16A. Also, the first regular intervals Λ1 of the gratings 103 are set to 1.98 µm, and a total length of a series of gratings 103 is 5 mm in a propagation direction of the fundamental waves P1. In addition, the gratings 103 are not arranged in the neighborhood of the incident end facet 83a (called a wavelength changing region 104) but arranged in the neighborhood of the output end facet 83b (called a wave reflecting region 105). The inverted-polarization layers 84 are arranged in not only the wavelength changing region 104 but also the wave reflecting region 105, and a total length of a series of layers 84 is 15 mm in the propagation direction of the fundamental waves P1. In the above configuration, 860 nm wavelength fundamental waves P1 radiated from the semiconductor laser 88 are radiated to the incident end facet 83a of the optical waveguide 83. Thereafter, a part of the fundamental waves P1 are changed to 430 nm wavelengthsecond fundamental waves P2 in the wavelength changing region 104. Therefore, the fundamental waves P1 not changed and the second harmonic waves P2 transmit through the optical waveguide 83 of the wave reflecting region 105. In the wave reflecting region 105, the fundamental waves P1 are not only changed to the second harmonic waves P2 but also selectively reflected by the gratings 103. In contrast, the second harmonic waves P2 are radiated from the output end facet 83b of the optical waveguide 83 without being reflected by the gratings 103. The reason that the fundamental waves P1 are selectively reflected by the gratings 103 is described with reference to Fig. 21. Fig. 21 is an enlarged cross-sectional view of the wave reflecting region 105 in the wavelength changing device 101 shown in Fig. 20A, intensity distributions of the fundamental waves P1 and the second fundamental waves P2 being explanatorily shown. The intensity distribution in a depth direction of coherent light transmitting through the optical waveguide 83 generally depends on the wavelength of the coherent light. As shown in Fig. 21 , an intensity distribution of the fundamental waves P1 spreads out to the dielectric passivation film 102 more than that of the harmonic waves P2. In detail, the intensity of the fundamental waves P1 is reduced to 1/e 2 of a maximum intensity of the waves P1 at a distance of 0.1 µm from an upper surface of the optical waveguide 83, and the intensity of the second harmonic waves P2 is reduced to 1/e 2 of a maximum intensity of the waves P2 at a distance of 0.04 µm from an upper surface of the optical waveguide 83. Therefore, because the dielectric passivation film 102 having the thickness D1=0.06 µm is arranged on the optical waveguide 83, the intensity distribution of the second harmonic waves P2 does not substantially spread out to the gratings 103. That is, the second harmonic waves P2 are not reflected by the gratings 103. In contrast, because the intensity distribution of the fundametal waves P1 spreads out to the gratings 103, the fundamental waves P1 are selectively reflected by the gratings 103. Thereafter, the fundamental waves P1 reflected by the gratings 103 are fed back to the semiconductor laser 88 in cases where the DBR condition Λ1 = mλ f /2N is satisfied. Here a refractive index N of the optical waveguide 83 is 2.17. Relation between the wavelength λ f of the fundamental waves P1 and the regular intervals Λ1 of the gratings 103 is shown in Fig. 22. As shown in Fig. 22, the 860 nm wavelength fundamental waves P1 are reflected by the gratings 103 functioning as the distributed Bragg reflector at the grating order m=10 because the first regular intervals Λ1 of the gratings 103 is 1.98 µm. Accordingly, even though the gratings 103 are arranged in the rear of the wavelength changing region 104, the second harmonic waves P2 are not reflected by the gratings 103 because the dielectric passivation film 102 is arranged between a series of gratings 103 and the optical waveguide 83. Therefore, the position of the gratings 103 is not limited to the front of the wavelength changing region 104 so that the wavelength changing device 101 can be arbitrarily manufactured. Also, the second fundamental waves P2 can be obtained at high efficiency. Also, because the fundamental waves P1 are changed to the second harmonic waves P2 in the wave reflecting region 105. the second fundamental waves P2 can be moreover obtained at high efficiency. 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. Optical characteristics of the wavelength changing device 101 are as follows. When the pumping power of the fundamental waves P1 radiated from the semiconductor laser 88 is 200 mW to couple the fundamental waves P1 to the optical waveguide 83 at a power of 120 mW (or a coupling efficiency is 60 %), the intensity of the harmonic waves P2 is 7 mW. Relation between the output power of the harmonic waves P2 and the thickness D1 of the dielectric passivation film 102 is described with reference to Fig. 23. As shown in Fig. 23, in cases where the thickness D1 of the dielectric passivation film 102 is increased to over 0.1 µm, the wavelength λ f of the fundamental waves P1 radiated from the semiconductor laser 88 is not fixed because the fundamental waves P1 are not reflected by the gratings 103. As a result, the second fundamental waves P2 cannot be stably obtained. Also, in cases where the thickness D1 of the dielectric passivation film 102 is decreased to less than 0.04 µm, the second harmonic waves P2 are scattered by the gratings 103. Therefore, as the thickness D1 of the dielectric passivation film 102 is decreased, the output power of the second harmonic waves P2 is decreased. Accordingly, in cases where the thickness D1 of the dielectric passivation film 102 ranges from 0.04 to 1.0 µm, the output power of the second harmonic waves P2 is equal to a maximum value of 7 mW. In general, the range of the thickness D1 is inversely proportional to a refractive index n of the dielectric passivation film 102. Therefore, in cases where a product of the reflactive index n and the thickness D1 ranges from 0.06 to 0.15 (0.06 < nD1(µm) < 0.15), the output power of the second harmonic waves P2 is maximized. In the embodiment, the dielectric passivation film 102 is made of SiO 2 . However, the material of the dielectric passivation film 102 is not limited. That is, any dielectric material can be utilized to make the dielectric passivation film 102 on condition that coherent light transmitting through the dielectric material is not substantially absorbed nor scattered. Also, the substrate 82 is made of pure LiTaO 3 material. However, the material of the substrate 82 is not limited to the pure LiTaO3 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 or LiTa (1-x) Nb x O 3 (0 ≤ x ≤ 1) be utilized to make the substrate 82. In addition, because KTiOPO 4 is a highly non-linear 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 gratings 103. A second 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 wavelength changing device 101 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 second 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 101, 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 101, 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, 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, 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. Having illustrated and described the principles of our invention in a preferred embodiment thereof, it should be readily apparent to those skilled in the art that the invention can be modified in arrangement and detail without departing from such principles. We claim all modifications coming within the scope of the accompanying claims.

Claims: A wavelength changing device, comprising: a substrate (82) made of a non-linear crystal, the substrate being polarized in a first polarization direction; an optical waveguide (83) arranged in the substrate for transmitting fundamental waves in a propagation direction perpendicular to the first polarization direction; and a plurality of inverted polarization layers (84) polarized in a second polarization direction opposite to the first polarization direction of the substrate and periodically arranged in the substrate at regular intervals in the propagation direction to change a part of the fundamental waves to second harmonic waves in alternate rows of the optical waveguide and the inverted polarization layers periodically crossing the optical waveguide; a dielectric film (102) arranged on the optical waveguide for confining the second harmonic waves produced in the alternate rows to prevent the second harmonic waves spreading outside the optical waveguide; and a plurality of grating elements (103) periodically arranged on the dielectric film at grating intervals in the propagation direction of the fundamental waves for reflecting the remaining part of the fundamental waves spreading outside the dielectric film. A laser beam generating apparatus including a wavelength charging device according to claim 1, and further comprising: a semiconductor laser (88) for radiating a beam of the fundamental waves to an incident end of the optical waveguide, the fundamental waves being reflected by the grating elements toward the semiconductor laser to fix the wavelength of the fundamental waves radiated from the semiconductor laser. A laser beam generating apparatus according to claim 2 in which a product of the thickness T µm of the dielectric film and the effective index is in the range of from 0.06 to 0.15 (0.06 ≤ nT ≤ 0.15). A laser beam generating apparatus according to claim 2 or 3 in which the dielectric film is made of a material selected from the group consisting of SiO 2 , Ta 2 O 5 , Ti 2 O 5 , SiN and LiNbO 3 . A device or apparatus according to any one of claims 1 to 4, in which the substrate is made of LiTa (1-X) Nb X O 3 (0 ≤ X ≤ 1). A device or apparatus according to any one of claims 1 to 5, in which the optical waveguide has a composition determined by exchanging some of the 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: G02B6, G02F1, H01S5