Abstract:
A second harmonic wave-generating element for generating a second harmonic wave from a fundamental wave, having an optical waveguide layer made of first epitaxial material having a fundamental composition of K 3 Li 2−x (Nb 1−Y Ta Y ) 5+X O 15+Z , an underclad part made of second epitaxial material having a fundamental composition of K 3 Li 2−X+A (Nb 1−Y−B Ta Y+B ) 5+X−A O 15+Z , an overclad part made of third epitaxial material having a fundamental composition of K 3 Li 2−X+C (Nb 1−Y−D Ta Y+D ) 5+X−C O 15+Z  and formed on and contacting the optical waveguide layer, wherein X=0.006 to 0.5, Y=0.00 to 0.05, A=0.006 to 0.12, B=0.005 to 0.5, C=0.006 to 0.12, D=0.005 to 0.5, X−A≦0, X−C≧0, |A−C|≦0.006, and |B−D|≦0.005).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a second harmonic wave-generating (SHG) element preferably usable for a device such as a blue laser source. 
     2. Related Art Statement 
     An element to generate a blue laser is suggested which is made by forming an optical waveguide having periodically polarization-inversed structure and in which an infrared semiconductor laser is introduced into the optical waveguide (U.S. Pat. No. 4,740,265, JP-A-5-289131, and JP-A-5-173213). For example, JP-A-6-51359 discloses a SHG element in which a polarization-inversed layer, an optical waveguide, a dielectric film, and a reflective grating layer are formed and a thickness of the dielectric film is defined into a given value. 
     Although these techniques require high-precisely controlling of domains, it is very hard to do so. An allowable temperature for the phase-matching must be controlled within a precision range of ±0.5° C. Moreover light damage of the optical waveguide may be recognized at 3 mw and over of a light energy. Considering these phenomena, it is pointed out that these techniques have a problem as a practical device. 
     On the other hand, NGK Insulators, Ltd. suggested in JP-A-8-339002 a SHG element having little light damage without a quasi-phase-matching or controlling domains at a high-precision. In this literature, a single crystal substrate is made of lithium potassium niobate or Ta-substituted lithium potassium niobate by micro pull-down method, and an optical waveguide made of a material of the same kind as of that of the substrate is formed on the substrate. 
     The SHG element was an epoch-making element because the element could make light damage remarkably small in the optical waveguide for converting a wavelength, and thereby an outlook for providing an element for a practical use can be obtained. However, for wide use as a blue laser source, it is required that a generation efficiency of a second harmonic wave be further increased. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to further develop a generation efficiency of a second harmonic wave. It is also an object to make light damage much smaller. 
     This invention relates to a second harmonic wave-generating element for generating a second harmonic wave from a fundamental wave, comprising an optical waveguide layer made of first epitaxial material having a fundamental composition of K 3 Li 2−X (Nb 1−Y Ta Y ) 5+X O 15+Z , an underclad part made of second epitaxial material having a fundamental composition of K 3 Li 2−X+A (Nb 1−Y−B Ta Y+B ) 5+X−A O 15+Z , an overclad part made of third epitaxial material having a fundamental composition of K 3 Li 2−X+C (Nb 1−Y−D Ta Y+D ) 5+X−C O 15+Z  and formed on and contacting the optical waveguide layer, wherein X=0.006 to 0.5, Y=0.00 to 0.05, A=0.006 to 0.12, B=0.005 to 0.5, C=0.006 to 0.12, D=0.005 to 0.5, X−A≧0, X−C≧0, |A−C|≦0.006, and |B−D|≦0.005). 
     This invention will be still more described with reference to FIG.  1 . 
     According to the investigation of the inventors, in a structure in which a single-layered three-dimensional optical waveguide is formed on a single-crystal substrate, an integrated value of an overlapped portion of a fundamental mode between a fundamental wave and a second harmonic wave, is small, whereby a high converting efficiency can not be obtained. In the case that a film made of another material such as a dielectric material (SiO 2 , Ta 2 O 5 ) is formed on the three-dimensional optical waveguide, the above integrated value of the overlapped portion could little improved, because the refractive index of the dielectric film is quite different from that of the three-dimensional optical waveguide. 
     In this invention, the inventors conceived the structure as schematically shown in FIG. 1 in which an optical waveguide layer  2 A is sandwiched between an underclad part  1  and an overclad part  4 A and controlling, as above mentioned, each of the fundamental compositions of the optical waveguide layer, the underclad part, and the overclad part. Consequently, they found that in a wavelength range of a light capable of generating a blue laser, particularly preferably a range in which the wavelength of a light to be phase-matched, is 780 nm to 940 nm, the optical waveguide becomes a single mode (fundamental mode)-optical waveguide and the mode-overlapping integration between the fundamental wave and the second harmonic wave become large, whereby the generation efficiency of the second harmonic wave become conspicuously large. 
     The second harmonic wave-generating element according to the present invention can generate a laser of a range of 390 nm to 470 nm, for example. Thus, it can be widely used as a device for optical disk memory, a medicine uses, an optochemical uses, various optical measurements, etc. by using such a laser having a short wavelength. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of this invention, reference is made to the attached drawings, wherein: 
     FIG. 1 is a cross sectional view schematically showing a main part of a second harmonic wave-generating device  1  of the present invention, 
     FIG. 2 is a cross sectional view schematically showing a preferred embodiment of a structure of an optical waveguide, an underclad part, and an overclad part, 
     FIG. 3 is a plan view schematically showing an element  11 C in a preferred embodiment of the present invention, 
     FIG. 4 is a side view schematically showing a preferred embodiment in the element  11 C of FIG. 3, 
     FIG.  5 ( a ) is a perspective view showing a part of an assembly before a thin film-heater and a dielectric layer are formed in the element  11 C of FIG. 4, 
     FIG.  5 ( b ) is a perspective view showing a part of an assembly after the dielectric layer and the thin film-heater are formed on the assembly of FIG.  5 ( a ), and 
     FIG. 6 is a transverse sectional view of a part of the element  11 C of FIG.  5 ( b ). 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In each of the fundamental composition, “X” is 0.006 to 0.5 (particularly preferably 0.006 to 0.02). “Y”, a ratio of substituted Ta to Nb, 0.00 to 0.05 (particularly preferably 0.00 to 0.01). 
     “A” and “C” are each 0.006 to 0.12, particularly preferably 0.006 to 0.02. The ranges of “A” and “C” show that in the fundamental composition of the underclad part and the overclad part, the amount of lithium contained therein is more than that in the optical waveguide and the total amount of niobium and tantalum contained therein is less than that in the optical waveguide, respectively. The range of “B” and “D”, which show that in the fundamental compositions of the underclad part and the overclad part, the amount of tantalum contained therein is more than that in the optical waveguide, respectively, is 0.005 to 0.5, particularly preferably 0.03 to 0.1. By controlling “A”, “B”, “C”, and “D”, a respective refractive index of each of the optical waveguide, the underclad part, and the overclad part can appropriately controlled. 
     “X−A” and “X−C” are not less than 0, particularly preferably not less than 0.01. 
     Moreover, for minimizing the integrated value of the overlapped portion, it is important to control the deviation between “A” and “C” and that between “B” and “D” with respect to the fundamental composition of the overclad part and the underclad part to not more than a given part value. Concretely, the difference between “A” and “C” has to be not more than 0.006, particularly preferably not more than 0.003, and the difference between “B” and “D” has to be not more than 0.005, particular preferably not more than 0.002. 
     The underclad part may be composed of a single crystal substrate or an epitaxial film formed on such a single crystal substrate. 
     The epitaxial material of each of the fundamental compositions is a single crystal or an orientated film. 
     As the fundamental materials of the optical waveguide layer, the underclad part, and the overclad part, a material having a tungsten bronze structure, made of K, Li, Nb, Ta, O can be employed (called as a “KLNT material”, hereinafter). However, within the composition range enabling the structure to be maintained, each element may be partially substituted. For example, K or Li may be partially substituted for Na, Rb, etc. 
     In a preferred embodiment, as schematically shown in the sectional view in FIG. 1, the optical waveguide layer is a three -dimensional optical waveguide  2 A, which is formed on a surface  1   a  of an underclad part  1 . An upper surface  2   a  of the three-dimensional optical waveguide  2 A is covered with an overclad layer  4 A. 
     Furthermore, in this embodiment, a width “m” and a height “n” of the three-dimensional optical waveguide  2 A may be 3.0 μm to 10.0 μm and 0.5 μm to 5.0 μm, respectively. Thereby, a single mode travelling can be realized and a propagation loss can be reduced. 
     Moreover, side faces  2   b  of the three-dimensional optical waveguide  2 A as viewed in a transverse section may be covered with a sideclad part  5 A, which may be composed of an epitaxial material having the same composition as that of the overclad part  4 A. Accordingly, in FIG. 1, an integral clad part  3 A is composed of the overclad part  4 A and the sideclad part  5 A. 
     Thereby, a propagation loss of a light travelling the three-dimensional optical waveguide can be further reduced to still more develop the output of a second harmonic wave. 
     Furthermore, as an element  11 B schematically shown in FIG. 2, a tilted angle θ of a side face  2   b  of a three-dimensional optical waveguide  2 B to the surface  1   a  of the underclad part  1  to may be smaller than 90 degree, concretely, 60 degree to 120 degree. This tilted angle influences the propagation loss. 
     Hereupon, in FIG. 2, an upper surface  2   a  of the three-dimensional optical waveguide  2 B is covered with an overclad part  4 B and the side faces  2   b  of the optical waveguide  2 B are covered with sideclad parts  5 B. An integral clad part  3 B is composed of the overclad part and the sideclad part. 
     Such parts of the surface  1   a  of the single crystal substrate  1  as not covered with the optical waveguide  2 B are covered with a film  6  made of the same KLNT material as that of the overclad part and the sideclad part. 
     The single crystal substrate may be preferably made by a micro pull-down method, which is suggested by the inventors in the JP-A-8-259375 and the JP-A-8-319191. 
     The optical waveguide layer, the overclad part, the underclad part and the sideclad part may be formed by a metalorganic vapor phase epitaxial method or a liquid phase epitaxial method. 
     The second harmonic wave-generating element may further comprise a reflective grating part for fixing the wavelength of a fundamental wave entering the optical waveguide and a temperature controlling means for controlling a temperature of at least the optical waveguide. 
     FIG. 3 to FIG. 6 show an embodiment in which the reflective grating part and the optical waveguide are formed on an integral substrate. FIG. 3 is a plan view schematically showing a part of a second harmonic wave-generating element  11 C in this embodiment. 
     The second harmonic wave-generating element  11 C has a single crystal substrate  12  having, for example, a rectangular parallelepiped shape. Formed on a surface of the substrate  12  are a three-dimensional optical waveguide  2 C and a reflective grating part  15 , on which a film heater  14  is formed. Hereupon, FIG. 3 shows positions of  2 C,  14 , and  15  in plane. A reference numeral  13  denotes a laser source. 
     A fundamental wave (an ordinary ray)  16  is led into the second harmonic wave-generating device  11 C from an incident end  12   a  of the substrate  12 . Thereafter, the fundamental wave  16  is led into the optical waveguide  2 C and passes the reflective grating part  15 . During the fundamental wave passing the part  15 , the wavelength of the fundamental wave is fixed with returning its light wave from the part  15 . Since the refractive index of an ordinary ray in the optical waveguide  2 C under the reflective grating part  15  does not almost change when the heater  14  generates a heat, the fixed wavelength not much influence the optical power. Moreover, the refractive index of an extraordinary ray in the optical waveguide  2 C can be increased by working the film heater  14 . Thereby, a wavelength of a second harmonic wave  17  can be controlled dynamically, and an output of the second harmonic wave  17  can be increased and optimized. A reference numeral  18  denotes an ordinary ray leaving from an end  12   b  of the substrate. 
     For example, if the environmental temperature decreases, the entire temperature in the optical waveguide  2 C decreases and the refractive index of an extraordinary ray decreases, even though the heating value of the film heater  14  is constant. If the decrease of the output of the second harmonic wave is detected, by rising the voltage in the film heater  14 , the entire temperature in the optical waveguide  2 C can be increased and thereby the refractive index of an extraordinary ray can be increased. 
     However, if a temperature in the optical waveguide at which a wavelength of the fundamental wave matches that of a second harmonic wave, is lower than an environmental temperature, it is likely to be difficult to carry out the above controlling method. Thus, the temperature in the optical waveguide  2 C at which a wavelength of the fundamental wave matches that of a second harmonic wave, is preferably higher than a maximum temperature of an environmental temperature by 10° C. and over. 
     On the other hand, a film-like heat-absorbing member such as a Peltier element instead of the film heater may be used. If the temperature in the wavelength-converting optical waveguide deviates the temperature in which the fundamental wave phase-matches the second harmonic wave, the Peltier element is operated so that the optical waveguide&#39;s temperature may return to the phase-matching temperature. 
     Next, a preferred embodiment of an element in FIG. 3 will be described with reference to FIG. 4 to FIG.  6 . FIG. 4 is a side view schematically showing the preferred embodiment of the second harmonic wave-generating element  11 C, FIG.  5 ( a ) is a perspective view showing a part of an optical waveguide in an enlarged scale, FIG.  5 ( b ) is a perspective view showing the same part as that of FIG.  5 ( a ) in a state after forming a dielectric layer and a film heater, and FIG. 6 is a transverse sectional view of the element of FIG.  5 ( b ). 
     A ridge-type optical waveguide  2 C is formed on a surface of a single crystal substrate  12  and a overclad layer  4 C is formed on the upper surface  2   a  of the optical waveguide. Ditches forming a diffraction grating at uniform period are formed on the upper surface portion of the overclad layer  4 C by reactive ion etching, for example, to compose a reflective grating part  15 . 
     A dielectric layer  20  is formed to cover the ridge-type optical waveguide  2 C and the overclad layer  4 C. The film heater  14  is formed in a given area on the dielectric layer  20 . A ridge structure  22  is composed of the optical waveguide  2 C, the overclad layer  4 C, and the dielectric layer  20 , and slender ditches  21  are formed in both the sides of the ridge structure  22 . 
     Although a material composing the dielectric layer is not limited, Ta 2 O 5 , SiO 2 , TiO 2 , HfO 2 , or Nb 2 O 5  is preferably employed. As a material composing the film heater, Ni, Ti, Ta, Pt, or Cr is preferably employed. Instead of the film heater, a Peltier element may be used. 
     EXAMPLE 
     The invention will be explained in more detail with reference to the following example. 
     In this example, an element as shown in FIG. 2 was produced. 
     A plate of a KLNT single crystal was formed by the micro pull-down method. Concretely, powdery potassium carbonate, lithium carbonate, niobium chloride, and tantalum oxide were mixed in a composition ratio of 30.0:24.0:45.0:0.92 to obtain a powdery raw material. About 50 g of the powdery raw material was put into a crucible made of platinum, and it was heated up to the temperature of 1150° C. to be melted. More concretely, the powdery raw material in the crucible was melted while a space in an upper side of a furnace was controlled to a temperature range of 1100° C. to 1200° C. A plate having a “C” crystal face was pulled down in a direction of an “a” crystal axis through a nozzle formed in a bottom face of the crucible at a speed of 10 mm/hour while a single crystal-growing part was set at a temperature range of 1050° C. to 1150° C. Consequently, a single crystal substrate could be grown in a thickness of 1 mm, a width of 30 mm, and a length of 30 mm. The substrate was used as an underclad part. The composition of the substrate was K 3 Li 2 (Nb 0.98 Ta 0.02 ) 5 O 15 . (0039) 
     Then, an epitaxial film was formed by the metalorganic vapor phase epitaxial method. Concretely, as starting materials, di-pivaloyl-methanato potassium [K(C 11 H 19 O 2 ) (called as “K(DPM)”, hereinafter)], di-pivaloyl methanato-lithium [Li(C 11 H 19 O 2 ) (called as “Li(DPM)”, hereinafter)], and penta-ethoxy niobium [Nb(OC 2 H 5 ) 5  (called as “Nb(PE)”, hereinafter)] were employed. They were charged into respective source containers, thereafter heated up to respective gasfication temperature to gasfy them. Each gas was introduced into a reactor chamber with use of a Ar carrier gas controlled at its flow rate. The flow rates of the gases were 250 ml/min for K(DPM), 500 ml/min for Li(DPM), 150 ml/min for Nb(PE), respectively. 
     In the conditions of the pressure in the reactor chamber being 20 torr and the temperature of the substrate being 650° C., a single crystal film having the thickness of 3.2 μm, made of a KLN material, was obtained by film-forming for 3 hours. The composition of the thus obtained film was K 3 Li 1.95 Nb 5.0.5 O 15 . 
     A strip-like film pattern of Ti was formed in a thickness of 1 μm, a width of 5 μm, a length of 25 mm, and a pitch of 2 mm by a normal photolithography. The thus obtained sample was processed by a reactive ion etching method. In this case, the sample was processed at a RF electric power of 250 W for 100 minutes with C 2 , F 6 , and O 2  gases under the pressure of 0.02 torr to form a ridge-type optical waveguide  2 B as shown in FIG.  2 . The width “p” of an upper surface  2   a , the width “m” of the lower surface, and the height “n” of the optical waveguide  2 B were 4 μm, 6 μm, and 2.5 μm, respectively. The tilted angle θ was 63 degree. 
     A clad part  3 B and a film  6  were formed on the sample by the metalorganic vapor phase epitaxial method as in the case of the optical waveguide  2 B. The flow rates of gasses were 200 ml/min for K(DPM), 600 ml/min for Li(DPM), 150 ml/min for Nb(PE), and 20 ml/min for penta-ethoxy tantalum [Ta(OC 2 H 5 ) 5 ]. In the conditions of the pressure in a reactor chamber being 20 Torr and the temperature of the substrate being 650° C., single crystal films  3 B and  6  having the thickness of 2.2 μm, made of a KLNT material, were obtained by film-forming for  2  hours. The composition of the thus obtained film was K 3 Li 1.99 (Nb 0.98 Ta 0.02 ) 5.01 O 15 . 
     The thus obtained sample was cut out to form chips in a length of 10 mm and a width of 2 mm as viewed in a direction of the optical waveguide. An input end and an output end of the element were optically polished, and coated with an anti-reflective film having a reflective index of 0.5% at a wavelength of 860 nm and an anti-reflective film having a reflective index of 0.5% at a wavelength of 430 nm, respectively. A titanium-sapphire laser was led into the element. Consequently, the laser was phase-matched at a wavelength of 862 nm to obtain a second harmonic wave having an output power of about 4 mW at 431 nm, when the input power from the laser was 100 mW. 
     As above mentioned, according to the present invention, an element having a high generation efficiency of a second harmonic wave can be obtained.