the present invention will now be described as embodied in an optical isolator . with the composite optical element defined in ( 1 ) above , the percentage of light beam that passes through the fifth and sixth regions are limited to 10 % or less . the limitation is a means of stabilizing its isolation ( reverse direction loss ) characteristics . this will be more concretely explained below . the light that has been transmitted in the reverse direction through the third birefringent region , faraday rotator , and first birefringent region ( percentage transmission k1 , phase δ1 ) and the light that has been transmitted in the reverse direction through the fourth birefringent region , faraday rotator , and second birefringent region ( percentage transmission k2 , phase δ2 ) do not travel straight as the phases are preset to be 1800 apart ( as noted above , optical path length =| 2 ( ne − no ) d |= λ / 2 ). in the mean time , the light that has been transmitted through the six region , faraday rotator , and fifth region ( percentage transmission k3 , phase δ3 ) goes straight in the absence of phase ( optical path length ) control . it is this straightly traveling light that deteriorates the isolation characteristics . ordinarily , with an optical isolator , at least 20 db isolation is believed to be necessary . restricting the percentage transmission of a light beam through the fifth and sixth regions to no more than 10 % renders it possible to attain at least 20 db isolation . the transmissivity of the light traveling straight in the reverse direction is | k1xe iδ1 k2xe iδ2 k1xe iδ3 . if k1 = k2 and | δ1 − δ2 |= 180 °, then the transmissivity is | k3 | 2 , thus if k3 ≦ 0 . 1 , the transmissivity ≦ 0 . 01 , and hence the isolation ( reverse direction loss ) ≧ 20 db . a composite optical element of such a construction can hardly be obtained by the conventional method ; to obtain it the process of the invention must be used . in ( 2 ) above , the refractive index of the fifth and sixth regions is set between the refractive indexes with respect to ordinary light and extraordinary light of those birefringent regions . the setting is for the sake of stability of the insertion loss ( forward direction loss ) characteristics . this will be more concretely explained below . the light that has been transmitted in the forward direction through the first birefringent region , faraday rotator , and second birefringent region ( percentage transmission k1 , phase δ1 ) and the light that has been transmitted in the forward direction through the third birefringent region , faraday rotator , and fourth birefringent region ( percentage transmission k2 , phase δ2 ) on the other hand , the light that has been transmitted through the fifth region , faraday rotator , and sixth region ( percentage transmission k3 , phase δ3 ) is diffracted with a loss when the phases ( optical path lengths ) differ . when the percentage transmission of a light beam through the fifth and sixth regions is limited to 10 % and when the refractive index ( na ) of the fifth and sixth regions is between the refractive indexes with respect to ordinary light ( no ) and extraordinary light ( ne ) of the birefringent regions , the resulting excess loss is at worst no more than 0 . 86 db ; practically the least necessary insertion loss of 1 db or less can thus be realized . the transmissivity of the light traveling straight in the forward direction is δk1xe iδ1 + k2x iδ2 + k1xe iδ3 . if k1 = k2 = 0 . 45 , k3 = 0 . 1 , and δ1 = δ2 , then the transmissivity is | 0 . 9 + 0 . 1xe i ( δ3 - δ1 ) | 2 . assuming that the overall thickness of the birefringent regions and the fifth and sixth regions is d , when na = no or na = ne , | δ3 − δ1 |= π / 2 and the transmissivity = 0 . 82 , or the transmission loss is 0 . 86 db . when na is midway between no and ne , | δ 3 − δ 1 ≦ π / 2 and the loss is ≦ 0 . 86 db and , especially when na =( no + ne )/ 2 , the loss = 0 db . ( 3 ) above is concerned with a process for producing the composite optical element defined in ( 1 ). [ 0114 ] fig2 through 8 illustrate the sequence of process steps for producing the composite optical element . in step 1 illustrated in fig2 a plurality each of a first quartz plate 11 and a second quartz plate 12 are prepared . their optical axes are along their planes a 1 and a 2 . in the quartz plate 11 the angle its optical axis makes to the plane b 1 is set to 22 . 5 ° and in the quartz plate 12 the angle its optical axis makes to the plane b 2 is set to 67 . 5 °. the two plates are made the same in thickness t1 and t2 , height b1 and b2 , and depth a1 and a2 . in step 2 shown in fig3 the pluralities of quartz plates 11 and 12 are joined , with the planes b 1 and b 2 face to face , to form a laminate . the plates are pressed altogether with an adequately strong pressure . in step 3 in fig4 the block of quartz plate so laminated is cut along the lines shown with arrows in two directions into a plurality of composite elements 13 of a desired thickness each , each element having a cut surface parallel with the planes a 1 and a 2 . in step 4 in fig5 either at least one cut surface of the composite element 13 to be joined to a faraday rotator or both cut surfaces are ground . the ground laminate is a composite element 14 . in step 5 in fig6 . one composite element 14 and another composite element turned in the opposite direction are joined to a faraday rotator 15 in between . bringing the left end or right end of these elements in contact with a reference surface r of a jig 16 allows the composite elements to be aligned precisely . in step 6 shown in fig7 the both planes of the composite optical elements 14 , 14 joined to the faraday rotator 15 in between are ground until the overall thickness reaches the designed thickness d . in step 7 shown in fig8 the joined laminate is cut into pieces of desired dimensions so that each cut piece is a composite optical element 10 consisting of a faraday rotator and a pair of composite elements joined to both sides of the rotator . light beam comes into the element 10 as a spot extending over the first and second regions . each such element 10 is diagrammatically shown on an enlarged scale as a front elevational view in ( a ) and as a side elevational view in ( b ) of fig1 . turning to fig1 the percentage of the light beam that is transmitted through the fifth and sixth regions is governed by the overall thickness of the adhesive layers used in joining the quartz plates 11 and 12 in step 2 . the thickness can be easily controlled through proper choice of factors such as the viscosity of the adhesive , and the degree of pressure that is applied for the joining purpose . the thickness can be reduced by using an adhesive of low viscosity and applying adequate pressure in joining . thus the process that follows the sequence of steps illustrated in fig2 to 8 produces a number of the composite optical elements of ( 1 ) above in a stable manner . the ( 4 ) above provides a process for producing the composite optical elements defined in ( 2 ) above . to be more specific , in the process ( 3 ) above , the refractive index after curing of the adhesive joining quartz plates 11 and 12 is set to a value between the refractive index with respect to ordinary light and the refractive index with respect to extraordinary light of the birefringent regions . the refractive indexes of the fifth and sixth regions are thus made equal . for this reason composite optical elements of low forward direction loss can be stably manufactured . the process ( 5 ) consists in setting the method of cutting and cut dimensions of quartz plates 11 and 12 as in step 1 of fig2 joining two composite elements 14 , one of which being turned in the reverse direction , and joining them to both sides of a faraday rotator , whereby the first and third birefringent regions , on one hand , and the second and fourth birefringent regions , on the other , are aligned respectively . in this way the number of components can be decreased . in the process ( 6 ), as illustrated in step 1 in fig2 the thickness t1 of the quartz plate 11 and the thickness t2 of the quartz plate 12 are set to be strictly the same . this permits the use of composite optical elements without the need of any adjustment . in the process ( 7 ), as represented by step 2 in fig3 the first birefringent region is joined to the second region and the third birefringent region to the fourth region without the use of adhesive but through optical contact . the optical contact is a phenomenon in which the planes b 1 and b 2 of the quartz plates 11 , 12 to be mated can be joined by grinding and polishing the two plane surfaces with superprecision instead of relying on adhesive . the process makes it possible to realize an ideal composite optical element free from any unwanted region . in ( 8 ) above , an optical isolator is made from one of the various composite optical elements according to the present invention . as illustrated in either fig9 or 10 , a permanent magnet 17 is disposed around a composite optical element 18 having two or more alternate regions of first and second birefringent regions and two or more alternate regions of third and fourth birefringent regions . ( no such permanent magnet is required where the faraday rotator is of a spontaneously magnetized material .) on both sides of the element are arranged a pair of collimator lenses 20 , 20 and farther outwardly incident photometric fiber 19 and emergence photometric fiber 19 are arranged as aligned to each other . in ( 9 ) above , an optical isolator is made from one of the various composite optical elements according to the present invention . as illustrated in fig1 , a permanent magnet 17 is disposed around a composite optical element 18 having two or more alternate regions of first and second birefringent regions and two or more alternate regions of third and fourth birefringent regions . ( no such permanent magnet is required where the faraday rotator is of a spontaneously magnetized material .) on both sides of the element are arranged a pair of collimator lenses 20 , 20 as aligned to each other . in ( 10 ) above , an optical isolator is made using one of the various composite optical elements according to the present invention . here optical fibers are arranged inside a capillary formed in a substrate , and the composite optical component is disposed inside a groove formed in the capillary . capillary types are taught in japanese patent application kokoku no . 7 - 109445 and japanese patent no . 2 , 683 , 266 . an example of the type is illustrated in fig1 . fig1 ( a ) is a side view and fig1 ( b ) is a plan view . a capillary ( fiber fixing member ) 31 is a cylinder of a solid material such as glass or ceramic . while termed capillary here , it is also known as ferule . the capillary has an axial hole 35 [ sic ] in which optical fibers 35 are fitted and held in place . a groove 38 is formed across the optical fiber , and a composite optical element 36 according to the present invention is fitted into the groove and fixed securely with adhesive . as another form of optical isolator according to the invention , as defined in ( 15 ) above , a composite optical element may be located with an inclination to the optical axis . fig1 shows an example . the isolator in fig1 has an inclined groove 39 with an inclination to the optical axis so as to prevent the return of light due to reflection at fiber ends that occurs in the isolator of fig1 , and a composite optical element 37 according to the invention is accommodated in the groove . fig1 ( a ) is a side view and fig1 ( b ) is a plan view . an optical attenuator needs not have a fifth and a sixth regions . preferably it uses a desirable one of the composite optical elements according to the invention and combines it with a means for applying a variable magnetic field to it . now the principles of an optical isolator will be explained . in an optical isolator : it is assumed that the angle the optical axis of the first birefringent region makes to the optical axis of the third birefringent region is φ and the angle of faraday rotation of a faraday rotator is θ . of the light incident to a composite optical element , polarized light travels as ordinary light through the first birefringent region and as extraordinary light through the second birefringent region . the proportion of the polarized light that passes as extraordinary light through the third birefringent region and as ordinary light through the fourth birefringent region is expressed as cos 2 ( θ − φ ). the length of the optical path through which the light has been transmitted through the first and third birefringent regions is set to be equal to the length of the optical path through which the light has been transmitted through the second and fourth birefringent regions . the polarized light , therefore , proceeds straight instead of being diffracted . the proportion of the polarized light that travels as ordinary light through the third birefringent region and as extraordinary light through the fourth birefringent region is expressed as sin 2 ( θ − φ ). the length of the optical path through which the light has been transmitted through the first and third birefringent regions is set to be different by half wavelength from the length of the optical path through which the light has been transmitted through the second and fourth birefringent regions . the polarized light is , therefore , diffracted instead of proceeding straight . of the light incident to a composite optical element , polarized light travels as extraordinary light through the first birefringent region and as ordinary light through the second birefringent region . the proportion of the polarized light that passes as ordinary light through the third birefringent region and as extraordinary light through the fourth birefringent region is expressed as cos 2 ( θ − φ ). the length of the optical path through which the light has been transmitted through the first and third birefringent regions is set to be equal to the length of the optical path through which the light has been transmitted through the second and fourth birefringent regions . the polarized light , therefore , proceeds straight instead of being diffracted . the proportion of the polarized light that travels as extraordinary light through the third birefringent region and as ordinary light through the fourth birefringent region is expressed as sin 2 ( θ − φ ). the length of the optical path through which the light has been transmitted through the first and third birefringent regions is set to be different by half wavelength from the length of the optical path through which the light has been transmitted through the second and fourth birefringent regions . the polarized light is , therefore , diffracted instead of proceeding straight . after all , the transmissivity of light that is incident from one optical fiber to another optical fiber is , independent of the polarization of light , sin 2 ( θ − φ ). the angle θ of faraday rotation can be made variable by turning the magnetic field being applied to the faraday rotator variable . in this way the transmissivity or attenuation is made variable , and an optical attenuator independent of light polarization can thus be realized . as the means for applying a magnetic field , electromagnet alone , combination of electromagnet and permanent magnet , and various other means may be used . concrete magnetic field - applying means are disclosed in japanese patent no . 2 , 815 , 509 ( optical attenuator ), patent application kokai no . 9 - 61770 , etc . the optical attenuator according to the present invention can be made small and compact and is less polarization - dependent than existing attenuators because it does not use a polarizer that varies the optical path with polarization . by way of example , a bismuth - substituted rare earth iron - garnet film is used as a faraday rotator , a magnetic field greater than saturation magnetic field is applied by a permanent magnet , as a magnetic field - applying means , in the direction perpendicular to the film , and means is provided for applying a magnetic field by an electromagnet in the inner planar direction of the film . at φ = 100 °, the angle of saturated faraday rotation is set to be θs = 100 °. when current is not flown through the electromagnet , θ = ƒs = 100 °, and ideally the transmissivity is 1 and the attenuation is 0 db . as the current through the electromagnet is increased , θ decreases below 100 °, and therefore the transmissivity declines while the attenuation increases . when θ = 10 °, the attenuation is maximum ( ideally it is infinite but actually it is restricted by other factors such as the quenching ratio of the element ). in practical application of the optical attenuator defined in ( 11 ) above , either lenses and optical fibers are arranged before and after a composite optical element for optical attenuation as in ( 12 ) above or optical fibers are arranged as in ( 13 ) above . in these arrangements , either collimator lenses and optical fibers are arranged as illustrated in fig9 and 10 in connection with an optical isolator or optical fibers are disposed as in fig1 . the only exception is that an electromagnet capable of flowing a variable current is employed in place of a permanent magnet as means for applying a magnetic field to the faraday rotator . in ( 14 ) above , an optical attenuator is made from one of the various composite optical elements according to the invention , by locating optical fibers in a capillary and disposing the composite optical component in a groove formed in the capillary , in the same manner as has been explained in connection with fig1 showing an optical isolator . similarly , in a modified form of the optical attenuator of the invention , as in ( 16 ) above , the composite optical element may be located inclined with respect to its optical axis . an example of it is built in the same way as illustrated in fig1 for optical isolator . the only exception is that an electromagnet capable of producing a variable magnetic field is employed as the magnet . examples of optical isolators utilizing inclined incident light as defined in ( 17 ) and ( 18 ) above will now be explained with reference to fig1 ( comparative example ) and fig1 ( this invention ). in both figures , ( a ) is a side elevational view and ( b ) is a plan view . when the plane including the vector of inclined incident light and the normal vector of incident surface of the composite optical element is not parallel to the interfacial boundary between the first and second birefringent regions 1 ′, 2 ′ as in fig1 , the light travels while being diffracted through the composite optical element . for this reason the interfacial boundary between the first and second birefringent regions 1 ′, 2 ′ and that between the third and fourth birefringent regions 3 ′, 4 ′ must be shifted with respect to each other rather than be on the same plane . moreover , the interfacial boundaries too must be inclined to a precise degree . in the optical isolator according to the present invention , therefore , the plane including the vector of inclined incident light and the normal vector of the incident surface of the composite optical element is set to be parallel to the interfacial boundary between the first and second birefringent regions 1 , 2 . the setting enables the light to move straight in the composite optical element as in fig1 ( b ) whereas it is diffracted in the element in fig1 ( a ). the light that has passed the first and second birefringent regions 1 , 2 can farther travel , respectively , through the third and fourth birefringent regions . thus the interfacial boundary between the first and second birefringent regions 1 , 2 and that between the third and fourth birefringent regions 3 , 4 have only to be parallel to each other , whereby the manufacture of the composite optical elements is facilitated . [ 0155 ] fig1 shows an embodiment of the capillary type optical isolator of the invention using optical fibers each having a core bulge in the vicinity of a composite optical element . the arrangement enhances the efficiency of optical utilization of the first to fourth birefringent regions of the composite optical element . in fig1 , optical fibers having a core bulge 40 each are inserted into the hollow of a capillary 31 , and the composite optical element 37 of the invention is sandwiched between the bulges . a permanent magnet 17 is held around the composite optical element . the optical attenuators that utilize inclined incident light as defined in ( 19 ), ( 20 ) above can adopt the construction illustrated in fig1 for ( 17 ), ( 18 ). [ 0157 ] fig1 shows an embodiment of the capillary type optical attenuator of the invention using optical fibers each having a core bulge in the vicinity of a composite optical element . the arrangement enhances the efficiency of optical utilization of the first to fourth birefringent regions of the composite optical element . in fig1 , optical fibers having a core bulge 40 each are inserted into the hollow of a capillary 31 , and the composite optical element 37 of the invention is sandwiched between the bulges . an optical - attenuator magnetic circuit comprising an electromagnet 43 capable of applying a variable magnetic field and a pair of permanent magnets 41 , 42 located before and after the electromagnet is provided around the composite optical element . typical compositions of composite optical element and optical isolator fabricated by the sequence of process steps thus far illustrated in fig2 to 8 will now be explained . many quartz plates 11 , 12 each measuring 10 mm × 11 mm × 1 . 7 mm ( a × b × t ) were prepared . other details are as shown in step 1 that has already been explained with reference to fig2 . using an optical adhesive of low viscosity , quartz plates 11 and 12 were alternately placed with their side b facing the same direction and then joined under a sufficiently large pressure . the thickness of each adhesive layer was 1 μm or less . the refractive index of the adhesive after curing was 1 . 532 . ( although technical difficulties are involved , optical contact may be used in joining instead of using the adhesive , as already explained .) the quartz plate laminate thus joined into a mass was cut into pieces each measuring 0 . 8 mm in thickness , with the cut surface parallel to the plane a . a total of 11 composite elements 3 were thus obtained . each element was ground and polished on both sides to a thickness of 0 . 6 mm , and a composite element 4 was obtained . as a faraday rotator , a bismuth - substituted rare earth iron - garnet with an angle of faraday rotation of 45 ° was used . the faraday rotator measured 10 × 10 × 0 . 4 mm thick . out of two same composite elements 4 , one was turned from side to side and the two elements were joined to the both sides of the faraday rotator . the both sides of the laminate were ground and polished until each of the composite element portions joined to the faraday rotator was 46 μm thick . the laminate was cut into pieces each with dimensions of about 1 . 5 mm × 1 . 5 mm ( as seen in the direction of its optical axis ) so that composite optical elements each of which combines a faraday rotator and two composite elements could be obtained . each composite optical element thus fabricated was combined with a permanent magnet , and the combination was arranged with a parallel beam system composed of two aligned single - mode optical fibers 19 , 19 and two lenses 20 , 20 as illustrated in fig1 . the optical isolator characteristics of the arrangement were determined to be 0 . 1 db insertion loss and 30 db isolation . the product was confirmed to function as a polarization - independent , polarization - dispersion - free optical isolator . a similarly fabricated composite optical element was combined with an electromagnet , and a magnetic field applied to a faraday rotator was changed . the amount of transmitted light varied , and the combination was confirmed to work normally as an optical attenuator . with the composition described above , the present invention achieves the following advantageous effects . ( 1 ) ideal composite optical elements in which the proportions and refractive indexes of the fifth and sixth regions are controlled so that the optical characteristics are stabilized , and optical devices such as optical isolators and attenuators using the elements can be realized . ( 2 ) in accordance with the process of the invention the composite optical elements ( 1 ) above can be manufactured with good reproducibility and stability . ( 3 ) each element may be made of only two rather than four of birefringent plates . ( this corresponds to ( 5 ) above .) ( 4 ) the positions of the regions on both sides can be determined without adjustment . ( this corresponds to ( 6 ) above .) only if the thicknesses of the quartz plates 11 , 12 are precisely predetermined , the both side regions can be automatically decided at the time of joining in step 5 . ( 5 ) even in the case of inclined light incidence , an element in which the interfacial boundary between the first and second regions and that between the third and fourth regions are in the same plate can be used . this facilitates the manufacture of the elements .