the present invention will hereinbelow be described in further detail with reference to the accompanying drawings . with reference to fig1 an optical wavelength conversion device 10 is constituted by an optical fiber comprising a first cladding layer 12 and a core 11 formed of a nonlinear optical material and filled in a hollow region at the center of the first cladding layer 12 . as the nonlinear optical material , an organic nonlinear optical material having a nonlinear optical constant markedly larger than that of an inorganic material should preferably be used . as the organic nonlinear optical material , it is possible to use , for example , 2 - methyl - 4 - nitroaniline ( mna ), methanitroaniline ( mna ), 3 - methyl - 4 - nitropyridine - 1 - oxide ( pom ), or urea as disclosed in japanese unexamined patent publication no . 60 ( 1985 )- 250334 ; &# 34 ; nonlinear optical properties of organic and polymeric materials &# 34 ;, acs symposium series 233 , david j . williams , american chemical society , 1983 ; and &# 34 ; yuki hisenkei kogaku zairyo &# 34 ; ( organic nonlinear optical materials ), masao kato , et al ., cmc k . k ., 1985 . for example , mna has a wavelength conversion efficiency approximately 2 , 000 times the wavelength conversion efficiency of linbo 3 as an inorganic nonlinear optical material . therefore , in the case where the core 11 is formed by use of the organic nonlinear optical material , a laser beam having a short wavelength , in the blue region , can be obtained by generating a second harmonic from an infrared laser beam emitted as the fundamental wave from a popular small and low - cost semiconductor laser . on the other hand , the first cladding layer 12 is formed of a material having a refractive index lower than the refractive index of the aforesaid nonlinear optical material . a second cladding layer 13 formed of a material having a refractive index lower than the refractive index of the first cladding layer 12 is formed outside of the first cladding layer 12 . the method of making the optical wavelength conversion device 10 will hereinbelow be described with reference to fig4 by taking the case where the core 11 is formed of mna , the first cladding layer 12 is formed of pyrex glass , and the second cladding layer 13 is formed of polytrifluoroisopropyl methacrylate , as an example . first , a hollow glass fiber 12 &# 39 ; for forming the first cladding layer 12 is prepared . by way of example , the glass fiber 12 &# 39 ; has an outer diameter of approximately 100 μm , and a diameter of the hollow region is in a range of approximately 2 μm to 10 μm . then , mna 11 &# 39 ; is kept in a melt condition in a furnace or the like , and one end of the glass fiber 12 &# 39 ; is dipped in the molten mna 11 &# 39 ;. as a result , the molten mna 11 &# 39 ; enters the hollow region of the glass fiber 12 &# 39 ; by capillary action . in order to prevent the molten mna 11 &# 39 ; from decomposing , the temperature of the molten mna 11 &# 39 ; is kept at a value slightly higher than its melting point ( 132 ° c .). thereafter , the glass fiber 12 &# 39 ; is quenched to convert the mna 11 &# 39 ; in the hollow region into a polycrystalline condition . the glass fiber 12 &# 39 ; should more preferably be gradually drawn from the furnace maintained at a temperature higher than the melting point of the mna 11 &# 39 ;, with a temperature outside the furnace being maintained at a temperature lower than said melting point , thereby to convert the molten mna 11 &# 39 ; to the single crystal condition at the draw - out region to the exterior of the furnace . in this manner , the core 11 consisting of the mna in the markedly long single crystal condition having a uniform crystal orientation is formed , and therefore the optical wavelength conversion device 10 can be made substantially long . as is well known , the wavelength conversion efficiency of the optical wavelength conversion device of this type is proportional to the length of the optical wavelength conversion device , and the practical value of the optical wavelength conversion device increases as the length thereof increases . after the core 11 is filled in the hollow region of the glass fiber 12 &# 39 ; in the manner as mentioned above , polytrifluoroisopropyl methacrylate is applied to the surface of the glass fiber 12 &# 39 ;, thereby to form the second cladding layer 13 . by way of example , the formation of the second cladding layer 13 is carried out by dissolving 20 g of polytrifluoroisopropyl methacrylate in 1 liter of methyl ethyl ketone to prepare a coating solution , applying the coating solution to the surface of the glass fiber 12 &# 39 ;, and drying the coating solution applied on the surface of the glass fiber 12 &# 39 ;. then , the glass fiber 12 &# 39 ; is cut at both ends to an appropriate length , and the optical wavelength conversion device 10 as shown in fig1 is obtained . the optical wavelength conversion device 10 is used in the manner as shown in fig1 . specifically , a fundamental wave 15 is made to enter the core 11 from a core edge face 11a . a q - switch yag laser ( wavelength : 1 . 06 μm ) 16 is used as the fundamental wave generating means , and a laser beam ( fundamental wave ) 15 guided by an objective lens 17 is made to impinge upon the core edge face 11a , thereby to make the laser beam 15 enter the optical wavelength conversion device 10 . the fundamental wave 15 is converted by the mna constituting the core 11 to a second harmonic 15 &# 39 ; having a wavelength one half the wavelength of the fundamental wave 15 . the second harmonic 15 &# 39 ; advances inside of the optical wavelength conversion device 10 by repeating total reflection among the outer surface regions of the first cladding layer 12 . by way of example , phase matching is effected between the guided mode of the fundamental wave 15 through the core and the radiation mode of the second harmonic 15 &# 39 ; to the first cladding layer 12 ( in the case of cherenkov radiation ). a beam 15 &# 34 ; including the second harmonic 15 &# 39 ; is radiated out of a radiation edge face 10a of the optical wavelength conversion device 10 . the radiated beam 15 &# 34 ; is passed through a filter ( not shown ), and the second harmonic 15 &# 39 ; alone is thus taken up for usage . refractive indexes n1 , n2 and n3 of the mna , pyrex glass and polytrifluoroisopropyl methacrylate respectively constituting the core 11 , the first cladding layer 12 and the second cladding layer 13 will now be described below . by way of example , the refractive indices with respect to the yag laser beam having the wavelength of 1 . 06 μm and the second harmonic having a wavelength of 0 . 53 μm obtained therefrom are as shown below . ______________________________________ wavelength 1 . 06 μm wavelength 0 . 53 μm______________________________________n1 1 . 496 -- n2 1 . 464 1 . 475n3 1 . 41 1 . 42______________________________________ where the value of n1 = 1 . 496 is the effective refractive index . in this case , formula ( 4 ) mentioned above is satisfied since n . sub . 2 . sup . ω = 1 . 464 and n . sub . 3 . sup . 2ω = 1 . 42 . accordingly , the total reflection of the second harmonic 15 &# 39 ; and the aforesaid phase matching can be effected . also , in the case where the aforesaid second cladding layer 13 is provided , the surface of the first cladding layer 12 is prevented from clinging of foreign substances , direct contact of a device supporting means , and damaging . also , by the provision of the second cladding layer 13 , the physical strength of the optical wavelength conversion device 10 becomes high . another embodiment of the optical wavelength conversion device in accordance with the present invention will hereinbelow be described with reference to fig5 . an optical wavelength conversion device 20 shown in fig5 is of the optical waveguide type , and comprises glass substrates 22 , 22 as the first cladding layers , and an optical waveguide 21 formed therebetween by filling a nonlinear optical material . also , second cladding layers 23 , 23 are respectively formed on the outer surfaces of the glass substrates 22 , 22 . the optical waveguide 21 , the glass substrates 22 , 22 , and the second cladding layers 23 , 23 may be formed respectively of the aforesaid mna , pyrex glass and polytrifluoroisopropyl methacrylate . also , filling of the mna between the glass substrates 22 , 22 and the formation of the second cladding layers 23 , 23 may be carried out basically in the same manner as mentioned above with reference to the first embodiment . the method of forming the optical waveguide by filling the nonlinear optical material between the glass substrates 22 , 22 is described in detail in , for example , japanese patent application nos . 61 ( 1986 )- 159292 and 61 ( 1986 )- 159293 . in order to enter the fundamental wave into the optical waveguide type optical wavelength conversion device 20 , the fundamental wave may be made to directly impinge upon the device edge face in the same manner as mentioned with reference to the embodiment shown in fig1 . however , in the embodiment shown in fig5 entry of the fundamental wave and radiation of the second harmonic are effected by use of grating couplers . specifically , a linear grating coupler ( hereinafter referred to as lgc ) 24 for entry of the fundamental wave is formed on the inner surface of the upper glass substrate 22 in fig5 i . e . on the surface thereof facing the optical waveguide 21 . also , a focusing grating coupler ( hereinafter referred to as fgc ) 25 for radiation of the second harmonic is formed on the outer surface of the upper glass substrate 22 in a substantially spaced relation to the lgc 24 . effects of the optical wavelength conversion device 20 having the aforesaid configuration will be described below . by way of example , the aforesaid yag laser 16 is used as the fundamental wave generating means , and the laser beam ( fundamental wave ) 15 guided by a collimator lens 26 is made to impinge upon the lgc 24 via the second cladding layer 23 and the glass substrate 22 . in this manner , the laser beam 15 can be entered to the optical waveguide 21 . the second harmonic is produced from the laser beam 15 as the fundamental wave , and the light beam 15 &# 34 ; including the second harmonic is radiated out of the fgc 25 on the second cladding layer 23 and is converged to a single spot . also with the optical wavelength conversion device 20 shown in fig5 wherein the second cladding layers 23 , 23 are formed outside of the glass substrates 22 , 22 , the same effects as the provision of the second cladding layer 13 in the optical wavelength conversion device 10 shown in fig1 can be obtained . in the aforesaid embodiments , mna is used as the nonlinear optical material , and the effective refractive index thereof with respect to the fundamental wave having a wavelength of 1 . 06 μm is adjusted to 1 . 496 . in the case where the effective refractive index changes in accordance with differences in orientation of the single crystal and the kind of the nonlinear optical material , the first cladding layer and the second cladding layer having suitable refractive indices should be selected . specifically , the cladding glasses constituting the first cladding layer and having various refractive indices may be employed . as the second cladding layer , instead of polytrifluoroisopropyl methacrylate , it is also possible to employ a resin material such as a melamine resin , a polyester resin , an acrylic resin , a silicone resin , an epoxy resin , a polyvinyl chloride resin , a polyethylene resin , a polypropylene resin , a polyamide resin , an acetyl cellulose resin ; glass such as silica glass ; or a transparent oxide crystal such as al 2 o 3 .